[PATCH v6 0/4] vfio-pci/zdev: Improved zPCI Function Measurement Support

[PATCH v6 0/4] vfio-pci/zdev: Improved zPCI Function Measurement Support

[Omar Elghoul]

Hi,

This patch series improves support for function measurement for zPCI
passthrough devices on s390.

Changelog
=========
v5 -> v6:
* Patch 2/4:
  - Rework the FMB re-enablement code to reuse the same buffer again
  - Make the FMB buffer persistent once allocated for as long as the
    device's lifetime to accommodate an architectural quirk
* Patch 4/4:
  - Update the liveness check to use the new FMB enabled bool

v4 -> v5:
* Typo in the cover letter
* Swap the ordering of patches 3/4 and 4/4 to ease merging (i.e., to
  ensure the three s390 patches are ordered before the VFIO patch)
* Patch 2/4:
  - Drop the refactor of zpci_fmb_enable_device() and the separation of
    zpci_fmb_clear_iommu_ctrs() and zpci_fmb_do_enable()
  - Allocate a new buffer in zpci_fmb_reenable_device() rather than
    reusing the same buffer to avoid firmware edge cases

* Patch 3/4 (previously 4/4):
  - Avoid reading from userspace while holding kzdev_lock unnecessarily

* Patch 4/4 (previously 3/4):
  - Drop allowing usercopy of the FMB when initializing the kmem_cache
  - Avoid copying to userspace while holding fmb_lock unnecessarily
    - Restore the FMB bounce buffer to achieve this one
  - Clarify uAPI documentation and ensure it accurately describes the
    behavior of the VFIO features

v3 -> v4:
* Patch 2/4:
  - Replace mutex_lock/unlock in zpci_reenable_device() with a guard

* Patch 3/4:
  - Allow usercopy of the FMB when initializing its kmem_cache
  - Move the guard in vfio_pci_zdev_feature_fmb_enable() lower to only
    protect the FMB
  - Ensure vfio_pci_zdev_feature_fmb_enable() fails on double-enable for
    consistency with the documentation
  - Eliminate the bounce buffer in vfio_pci_zdev_feature_fmb_read()
  - Replace the void pointer with __aligned_u64 in the FMB read uAPI
    structure

v2 -> v3:
* Patch 1/4 (new patch):
  - Fix race conditions in pcibios_enable/disable_device() with regard to
    the FMB enable/disable
  - Assert that fmb_lock is held within zpci_fmb_enable_device() and
    zpci_fmb_disable_device()

* Patch 2/4 (previously 1/3):
  - Move the FMB enable logic into a static function zpci_fmb_do_enable()
    to reduce code duplication between zpci_fmb_enable_device() and
    zpci_fmb_reenable_device()
  - Reword commit message to use the imperative voice more consistently

* Patch 3/4 (previously 2/3):
  - Split the previous VFIO feature into a SET-only and a GET-only feature
    for enabling/disabling and reading the FMB respectively
  - Remove FMB definitions from the VFIO uAPI and instead treat it as an
    opaque structure

* Patch 4/4 (previously 3/3):
  - Clarify goto label name to reduce misunderstandings

v1 -> v2:
* Patch 1/3:
  - Address a possible race condition in zpci_reenable_device() caused by
    calling zpci_fmb_reenable_device() without holding fmb_lock
  - Assert that fmb_lock is held within zpci_fmb_reenable_device()

* Patch 3/3:
  - Address a possible race condition in pci_perf_seq_write() caused by
    consuming zdev->kzdev without holding kzdev_lock

Motivation
==========
The firmware on s390x machines allows for tracking a variety of statistics
relating to zPCI devices in a function measurement block (FMB). However,
the kernel currently lacks a structured mechanism of sharing this
information with userspace, beyond /sys/kernel/debug/pci/ID/statistics.
This can lead to shortcomings when running a guest on KVM with PCI
passthrough devices, as QEMU is unable to provide an accurate FMB snapshot
to the guest.

Proposal
========
We propose adding a new VFIO device feature to zPCI passthrough devices,
allowing userspace programs to read the latest FMB snapshot as it is
written by the firmware. We ensure that function measurement enablement is
preserved across device resets on the host. Furthermore, we guard against
host tampering with the FMB via sysfs when the zPCI device is in
passthrough to protect the VM's state.

I'd appreciate some feedback on these patches.

Thanks in advance.

Omar Elghoul (4):
  s390/pci: Hold fmb_lock when enabling or disabling PCI devices
  s390/pci: Reuse FMB buffer and preserve state in device re-enablement
  s390/pci: Fence FMB enable/disable via debugfs for passthrough devices
  vfio-pci/zdev: Add VFIO FMB device features

 arch/s390/include/asm/pci.h      |  2 +
 arch/s390/pci/pci.c              | 83 +++++++++++++++++++++++++-------
 arch/s390/pci/pci_debug.c        | 11 ++++-
 drivers/vfio/pci/vfio_pci_core.c |  4 ++
 drivers/vfio/pci/vfio_pci_priv.h | 18 +++++++
 drivers/vfio/pci/vfio_pci_zdev.c | 60 +++++++++++++++++++++++
 include/uapi/linux/vfio.h        | 29 +++++++++++
 7 files changed, 189 insertions(+), 18 deletions(-)

-- 
2.55.0

[PATCH v6 1/4] s390/pci: Hold fmb_lock when enabling or disabling PCI devices

[Omar Elghoul]

Ensure that fmb_lock is held by pcibios_enable_device() and
pcibios_disable_device() when calling zpci_fmb_enable_device() or
zpci_fmb_disable_device(), respectively. Additionally, assert that the
fmb_lock is held within the latter two functions to prevent future race
conditions regarding new callers.

Fixes: af0a8a8453f7 ("s390/pci: implement pcibios_add_device")
Fixes: 944239c59e93 ("s390/pci: implement pcibios_release_device")
Cc: stable@vger.kernel.org
Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
 arch/s390/pci/pci.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
index d64c544b32b0..c055a9ad0972 100644
--- a/arch/s390/pci/pci.c
+++ b/arch/s390/pci/pci.c
@@ -173,6 +173,8 @@ int zpci_fmb_enable_device(struct zpci_dev *zdev)
 	unsigned long flags;
 	u8 cc, status;
 
+	lockdep_assert_held(&zdev->fmb_lock);
+
 	if (zdev->fmb || sizeof(*zdev->fmb) < zdev->fmb_length)
 		return -EINVAL;
 
@@ -211,6 +213,8 @@ int zpci_fmb_disable_device(struct zpci_dev *zdev)
 	struct zpci_fib fib = {0};
 	u8 cc, status;
 
+	lockdep_assert_held(&zdev->fmb_lock);
+
 	if (!zdev->fmb)
 		return -EINVAL;
 
@@ -639,7 +643,9 @@ int pcibios_enable_device(struct pci_dev *pdev, int mask)
 	struct zpci_dev *zdev = to_zpci(pdev);
 
 	zpci_debug_init_device(zdev, dev_name(&pdev->dev));
+	mutex_lock(&zdev->fmb_lock);
 	zpci_fmb_enable_device(zdev);
+	mutex_unlock(&zdev->fmb_lock);
 
 	return pci_enable_resources(pdev, mask);
 }
@@ -648,7 +654,9 @@ void pcibios_disable_device(struct pci_dev *pdev)
 {
 	struct zpci_dev *zdev = to_zpci(pdev);
 
+	mutex_lock(&zdev->fmb_lock);
 	zpci_fmb_disable_device(zdev);
+	mutex_unlock(&zdev->fmb_lock);
 	zpci_debug_exit_device(zdev);
 }
 
-- 
2.55.0

Re: [PATCH v6 1/4] s390/pci: Hold fmb_lock when enabling or disabling PCI devices

[Matthew Rosato]

On 9/16/26 5:32 PM, Omar Elghoul wrote:
> Ensure that fmb_lock is held by pcibios_enable_device() and
> pcibios_disable_device() when calling zpci_fmb_enable_device() or
> zpci_fmb_disable_device(), respectively. Additionally, assert that the
> fmb_lock is held within the latter two functions to prevent future race
> conditions regarding new callers.
> 
> Fixes: af0a8a8453f7 ("s390/pci: implement pcibios_add_device")
> Fixes: 944239c59e93 ("s390/pci: implement pcibios_release_device")
> Cc: stable@vger.kernel.org
> Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
> Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>

[PATCH v6 2/4] s390/pci: Reuse FMB buffer and preserve state in device re-enablement

[Omar Elghoul]

Introduce the function zpci_fmb_reenable_device() that checks the state
of function measurement and ensures it is enabled. Reset the counters to
zero, disable, and re-enable the FMB if it was already enabled. Call
this function from zpci_reenable_device().

Don't free the FMB buffer during disabling and reuse it when re-enabling
measurement. Instead, free the buffer upon device teardown, allowing the
same buffer to be reused in the enable path and add the bit fmb_enabled
to struct zpci_dev. Audit the only consumer of zdev->fmb and update it
to reflect the change in semantics.

Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
---
 arch/s390/include/asm/pci.h |  2 +
 arch/s390/pci/pci.c         | 75 ++++++++++++++++++++++++++++---------
 arch/s390/pci/pci_debug.c   |  2 +-
 3 files changed, 61 insertions(+), 18 deletions(-)

diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
index 88a125b92bdd..b8162f7a8968 100644
--- a/arch/s390/include/asm/pci.h
+++ b/arch/s390/include/asm/pci.h
@@ -175,6 +175,7 @@ struct zpci_dev {
 	u8		util_str_avail	: 1;
 	u8		tid_avail	: 1;
 	u8		rtr_avail	: 1; /* Relaxed translation allowed */
+	u8		fmb_enabled	: 1;
 	unsigned int	devfn;		/* DEVFN part of the RID*/
 
 	u8 pfip[CLP_PFIP_NR_SEGMENTS];	/* pci function internal path */
@@ -351,6 +352,7 @@ void zpci_remove_parent_msi_domain(struct zpci_bus *zbus);
 /* FMB */
 int zpci_fmb_enable_device(struct zpci_dev *);
 int zpci_fmb_disable_device(struct zpci_dev *);
+int zpci_fmb_reenable_device(struct zpci_dev *zdev);
 
 /* Debug */
 int zpci_debug_init(void);
diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
index c055a9ad0972..5d1f5b75bff5 100644
--- a/arch/s390/pci/pci.c
+++ b/arch/s390/pci/pci.c
@@ -175,13 +175,18 @@ int zpci_fmb_enable_device(struct zpci_dev *zdev)
 
 	lockdep_assert_held(&zdev->fmb_lock);
 
-	if (zdev->fmb || sizeof(*zdev->fmb) < zdev->fmb_length)
+	if (zdev->fmb_enabled || sizeof(*zdev->fmb) < zdev->fmb_length)
 		return -EINVAL;
 
-	zdev->fmb = kmem_cache_zalloc(zdev_fmb_cache, GFP_KERNEL);
-	if (!zdev->fmb)
-		return -ENOMEM;
-	WARN_ON((u64) zdev->fmb & 0xf);
+	if (!zdev->fmb) {
+		zdev->fmb = kmem_cache_zalloc(zdev_fmb_cache, GFP_KERNEL);
+		if (!zdev->fmb)
+			return -ENOMEM;
+		WARN_ON((u64) zdev->fmb & 0xf);
+	} else {
+		/* FMB buffers are intentionally persistent for later reuse */
+		memset(zdev->fmb, 0, sizeof(*zdev->fmb));
+	}
 
 	/* reset software counters */
 	spin_lock_irqsave(&zdev->dom_lock, flags);
@@ -199,11 +204,11 @@ int zpci_fmb_enable_device(struct zpci_dev *zdev)
 	fib.fmb_addr = virt_to_phys(zdev->fmb);
 	fib.gd = zdev->gisa;
 	cc = zpci_mod_fc(req, &fib, &status);
-	if (cc) {
-		kmem_cache_free(zdev_fmb_cache, zdev->fmb);
-		zdev->fmb = NULL;
-	}
-	return cc ? -EIO : 0;
+	if (cc)
+		return -EIO;
+
+	zdev->fmb_enabled = 1;
+	return 0;
 }
 
 /* Modify PCI: Disable PCI function measurement */
@@ -215,7 +220,7 @@ int zpci_fmb_disable_device(struct zpci_dev *zdev)
 
 	lockdep_assert_held(&zdev->fmb_lock);
 
-	if (!zdev->fmb)
+	if (!zdev->fmb_enabled)
 		return -EINVAL;
 
 	fib.gd = zdev->gisa;
@@ -224,13 +229,39 @@ int zpci_fmb_disable_device(struct zpci_dev *zdev)
 	cc = zpci_mod_fc(req, &fib, &status);
 	if (cc == 3) /* Function already gone. */
 		cc = 0;
+	if (cc)
+		return -EIO;
 
-	if (!cc) {
-		kmem_cache_free(zdev_fmb_cache, zdev->fmb);
-		zdev->fmb = NULL;
-	}
-	return cc ? -EIO : 0;
+	zdev->fmb_enabled = 0;
+	return 0;
 }
+EXPORT_SYMBOL_GPL(zpci_fmb_disable_device);
+
+int zpci_fmb_reenable_device(struct zpci_dev *zdev)
+{
+	u64 req = ZPCI_CREATE_REQ(zdev->fh, 0, ZPCI_MOD_FC_SET_MEASURE);
+	struct zpci_fib fib = {0};
+	u8 cc, status;
+
+	lockdep_assert_held(&zdev->fmb_lock);
+
+	if (!zdev->fmb_enabled)
+		return zpci_fmb_enable_device(zdev);
+
+	fib.gd = zdev->gisa;
+	cc = zpci_mod_fc(req, &fib, &status); /* Disable function measurement */
+
+	/* Unlike in zpci_fmb_disable_device(), cc == 3 is not a valid state here
+	 * because we are re-enabling function measurement for the same function
+	 * handle.
+	 */
+	if (cc)
+		return -EIO;
+
+	zdev->fmb_enabled = 0;
+	return zpci_fmb_enable_device(zdev);
+}
+EXPORT_SYMBOL_GPL(zpci_fmb_reenable_device);
 
 static int zpci_cfg_load(struct zpci_dev *zdev, int offset, u32 *val, u8 len)
 {
@@ -737,9 +768,13 @@ int zpci_reenable_device(struct zpci_dev *zdev)
 	}
 
 	rc = zpci_iommu_register_ioat(zdev, &status);
-	if (rc)
+	if (rc) {
 		zpci_disable_device(zdev);
+		return rc;
+	}
 
+	guard(mutex)(&zdev->fmb_lock);
+	zpci_fmb_reenable_device(zdev);
 	return rc;
 }
 EXPORT_SYMBOL_GPL(zpci_reenable_device);
@@ -1003,6 +1038,12 @@ void zpci_release_device(struct kref *kref)
 	if (zdev->has_resources)
 		zpci_cleanup_bus_resources(zdev);
 
+	if (zdev->fmb) {
+		zdev->fmb_enabled = 0;
+		kmem_cache_free(zdev_fmb_cache, zdev->fmb);
+		zdev->fmb = NULL;
+	}
+
 	zpci_bus_device_unregister(zdev);
 	zpci_destroy_iommu(zdev);
 	zpci_dbg(3, "rem fid:%x\n", zdev->fid);
diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c
index c7ed7bf254b5..44f026ead414 100644
--- a/arch/s390/pci/pci_debug.c
+++ b/arch/s390/pci/pci_debug.c
@@ -97,7 +97,7 @@ static int pci_perf_show(struct seq_file *m, void *v)
 		return 0;
 
 	mutex_lock(&zdev->fmb_lock);
-	if (!zdev->fmb) {
+	if (!zdev->fmb_enabled) {
 		mutex_unlock(&zdev->fmb_lock);
 		seq_puts(m, "FMB statistics disabled\n");
 		return 0;
-- 
2.55.0

Re: [PATCH v6 2/4] s390/pci: Reuse FMB buffer and preserve state in device re-enablement

[Matthew Rosato]

> +int zpci_fmb_reenable_device(struct zpci_dev *zdev)
> +{
> +	u64 req = ZPCI_CREATE_REQ(zdev->fh, 0, ZPCI_MOD_FC_SET_MEASURE);
> +	struct zpci_fib fib = {0};
> +	u8 cc, status;
> +
> +	lockdep_assert_held(&zdev->fmb_lock);
> +
> +	if (!zdev->fmb_enabled)
> +		return zpci_fmb_enable_device(zdev);

Sashiko has a point here, won't all paths that call zpci_reenable_device
now unconditionally turn on FMB here even if it was purposely disabled?

The pattern of callers for zpci_reenable_device() is:

zpci_disable_device()
...
zpci_reenable_device()

We only care to re-enable the FMB during device re-enable if the device
had FMB enabled at the time zpci_disable_device() was called.

Thanks,
Matt

> +
> +	fib.gd = zdev->gisa;
> +	cc = zpci_mod_fc(req, &fib, &status); /* Disable function measurement */
> +
> +	/* Unlike in zpci_fmb_disable_device(), cc == 3 is not a valid state here
> +	 * because we are re-enabling function measurement for the same function
> +	 * handle.
> +	 */
> +	if (cc)
> +		return -EIO;
> +
> +	zdev->fmb_enabled = 0;
> +	return zpci_fmb_enable_device(zdev);
> +}
> +EXPORT_SYMBOL_GPL(zpci_fmb_reenable_device);

Re: [PATCH v6 2/4] s390/pci: Reuse FMB buffer and preserve state in device re-enablement

[Omar Elghoul]

On 9/16/26 10:21 PM, Matthew Rosato wrote:
> 
>> +int zpci_fmb_reenable_device(struct zpci_dev *zdev)
>> +{
>> +	u64 req = ZPCI_CREATE_REQ(zdev->fh, 0, ZPCI_MOD_FC_SET_MEASURE);
>> +	struct zpci_fib fib = {0};
>> +	u8 cc, status;
>> +
>> +	lockdep_assert_held(&zdev->fmb_lock);
>> +
>> +	if (!zdev->fmb_enabled)
>> +		return zpci_fmb_enable_device(zdev);
> 
> Sashiko has a point here, won't all paths that call zpci_reenable_device
> now unconditionally turn on FMB here even if it was purposely disabled?
> 
> The pattern of callers for zpci_reenable_device() is:
> 
> zpci_disable_device()
> ...
> zpci_reenable_device()
> 
> We only care to re-enable the FMB during device re-enable if the device
> had FMB enabled at the time zpci_disable_device() was called.

The way I thought of it was pcibios_enable_device() already enables
measurement, so we might as well ensure it is enabled when the zdev is
re-enabled. That default state is also helpful for the later patches
because kvm_s390_pci_register_kvm() calls zpci_reenable_device().

But I guess if we didn't want to conflate the two, we could also add an
unconditional zpci_fmb_reenable_device() within the KVM register hook,
to run after the zpci_reenable_device(), whichever you'd prefer.

Thanks

> 
> Thanks,
> Matt
> 
>> +
>> +	fib.gd = zdev->gisa;
>> +	cc = zpci_mod_fc(req, &fib, &status); /* Disable function measurement */
>> +
>> +	/* Unlike in zpci_fmb_disable_device(), cc == 3 is not a valid state here
>> +	 * because we are re-enabling function measurement for the same function
>> +	 * handle.
>> +	 */
>> +	if (cc)
>> +		return -EIO;
>> +
>> +	zdev->fmb_enabled = 0;
>> +	return zpci_fmb_enable_device(zdev);
>> +}
>> +EXPORT_SYMBOL_GPL(zpci_fmb_reenable_device);
> 

Re: [PATCH v6 2/4] s390/pci: Reuse FMB buffer and preserve state in device re-enablement

[Matthew Rosato]

On 9/17/26 9:32 AM, Omar Elghoul wrote:
> On 9/16/26 10:21 PM, Matthew Rosato wrote:
>>
>>> +int zpci_fmb_reenable_device(struct zpci_dev *zdev)
>>> +{
>>> +    u64 req = ZPCI_CREATE_REQ(zdev->fh, 0, ZPCI_MOD_FC_SET_MEASURE);
>>> +    struct zpci_fib fib = {0};
>>> +    u8 cc, status;
>>> +
>>> +    lockdep_assert_held(&zdev->fmb_lock);
>>> +
>>> +    if (!zdev->fmb_enabled)
>>> +        return zpci_fmb_enable_device(zdev);
>>
>> Sashiko has a point here, won't all paths that call zpci_reenable_device
>> now unconditionally turn on FMB here even if it was purposely disabled?
>>
>> The pattern of callers for zpci_reenable_device() is:
>>
>> zpci_disable_device()
>> ...
>> zpci_reenable_device()
>>
>> We only care to re-enable the FMB during device re-enable if the device
>> had FMB enabled at the time zpci_disable_device() was called.
> 
> The way I thought of it was pcibios_enable_device() already enables
> measurement, so we might as well ensure it is enabled when the zdev is

Yes, FMB starts enabled by default but then it could be disabled
intentionally via sysfs; if a recovery action occurs on the device
sometime after I manually disabled FMB for a device, I'd expect it to
still be disabled after the recovery action but AFAICT with this
codepath it will always get re-enabled.

Completely removing the device and re-introducing it to the system is a
different matter; in that case yeah we'd be back to FMB-enabled because
that's our default starting state via pcibios_enable_device().

> re-enabled. That default state is also helpful for the later patches
> because kvm_s390_pci_register_kvm() calls zpci_reenable_device().
> 
> But I guess if we didn't want to conflate the two, we could also add an
> unconditional zpci_fmb_reenable_device() within the KVM register hook,
> to run after the zpci_reenable_device(), whichever you'd prefer.

Do we even want it to be part of the KVM register hook at all, or could
we leave it off until we get a VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE?

That way if a guest isn't using FMB (we never intercepted the associated
MPCIFC to enable measurement), QEMU would never issue
VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE and then we just leave the host FMB off.

In practice, yes, FMB is always on by default for a linux guest today
but if we were to ever move to a model where we make it opt-in, it would
be nice to only enable the host FMB when the guest actually wants it.

Re: [PATCH v6 2/4] s390/pci: Reuse FMB buffer and preserve state in device re-enablement

[Omar Elghoul]

On 9/17/26 11:28 AM, Matthew Rosato wrote:
> On 9/17/26 9:32 AM, Omar Elghoul wrote:
>> On 9/16/26 10:21 PM, Matthew Rosato wrote:
>>>
>>>> +int zpci_fmb_reenable_device(struct zpci_dev *zdev)
>>>> +{
>>>> +    u64 req = ZPCI_CREATE_REQ(zdev->fh, 0, ZPCI_MOD_FC_SET_MEASURE);
>>>> +    struct zpci_fib fib = {0};
>>>> +    u8 cc, status;
>>>> +
>>>> +    lockdep_assert_held(&zdev->fmb_lock);
>>>> +
>>>> +    if (!zdev->fmb_enabled)
>>>> +        return zpci_fmb_enable_device(zdev);
>>>
>>> Sashiko has a point here, won't all paths that call zpci_reenable_device
>>> now unconditionally turn on FMB here even if it was purposely disabled?
>>>
>>> The pattern of callers for zpci_reenable_device() is:
>>>
>>> zpci_disable_device()
>>> ...
>>> zpci_reenable_device()
>>>
>>> We only care to re-enable the FMB during device re-enable if the device
>>> had FMB enabled at the time zpci_disable_device() was called.
>>
>> The way I thought of it was pcibios_enable_device() already enables
>> measurement, so we might as well ensure it is enabled when the zdev is
> 
> Yes, FMB starts enabled by default but then it could be disabled
> intentionally via sysfs; if a recovery action occurs on the device
> sometime after I manually disabled FMB for a device, I'd expect it to
> still be disabled after the recovery action but AFAICT with this
> codepath it will always get re-enabled.
> 
> Completely removing the device and re-introducing it to the system is a
> different matter; in that case yeah we'd be back to FMB-enabled because
> that's our default starting state via pcibios_enable_device().

Makes sense, I will fix this in the next version.

> 
>> re-enabled. That default state is also helpful for the later patches
>> because kvm_s390_pci_register_kvm() calls zpci_reenable_device().
>>
>> But I guess if we didn't want to conflate the two, we could also add an
>> unconditional zpci_fmb_reenable_device() within the KVM register hook,
>> to run after the zpci_reenable_device(), whichever you'd prefer.
> 
> Do we even want it to be part of the KVM register hook at all, or could
> we leave it off until we get a VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE?
> 
> That way if a guest isn't using FMB (we never intercepted the associated
> MPCIFC to enable measurement), QEMU would never issue
> VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE and then we just leave the host FMB off.
> 
> In practice, yes, FMB is always on by default for a linux guest today
> but if we were to ever move to a model where we make it opt-in, it would
> be nice to only enable the host FMB when the guest actually wants it.

[PATCH v6 3/4] s390/pci: Fence FMB enable/disable via debugfs for passthrough devices

[Omar Elghoul]

Introduce a fence over enabling or disabling FMB via debugfs when the zPCI
device is associated with a KVM. This will prevent processes on the host
from tampering with the FMB while the guest is still using it, which may
cause partial counter resets and inconsistent reads which have no parallel
in the architecture.

For VFIO devices that are not associated with a KVM (i.e., for userspace
drivers other than QEMU), this fence does not take effect.

Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
---
 arch/s390/pci/pci_debug.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c
index 44f026ead414..afc52bd5271a 100644
--- a/arch/s390/pci/pci_debug.c
+++ b/arch/s390/pci/pci_debug.c
@@ -153,6 +153,12 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
 	if (rc)
 		return rc;
 
+	mutex_lock(&zdev->kzdev_lock);
+	if (zdev->kzdev) {
+		rc = -EPERM;
+		goto out_unlock_kzdev;
+	}
+
 	mutex_lock(&zdev->fmb_lock);
 	switch (val) {
 	case 0:
@@ -163,6 +169,9 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
 		break;
 	}
 	mutex_unlock(&zdev->fmb_lock);
+
+out_unlock_kzdev:
+	mutex_unlock(&zdev->kzdev_lock);
 	return rc ? rc : count;
 }
 
-- 
2.55.0

Re: [PATCH v6 3/4] s390/pci: Fence FMB enable/disable via debugfs for passthrough devices

[Matthew Rosato]

On 9/16/26 5:32 PM, Omar Elghoul wrote:
> Introduce a fence over enabling or disabling FMB via debugfs when the zPCI
> device is associated with a KVM. This will prevent processes on the host
> from tampering with the FMB while the guest is still using it, which may
> cause partial counter resets and inconsistent reads which have no parallel
> in the architecture.
> 
> For VFIO devices that are not associated with a KVM (i.e., for userspace
> drivers other than QEMU), this fence does not take effect.
> 
> Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>

> ---
>  arch/s390/pci/pci_debug.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c
> index 44f026ead414..afc52bd5271a 100644
> --- a/arch/s390/pci/pci_debug.c
> +++ b/arch/s390/pci/pci_debug.c
> @@ -153,6 +153,12 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
>  	if (rc)
>  		return rc;
>  
> +	mutex_lock(&zdev->kzdev_lock);
> +	if (zdev->kzdev) {
> +		rc = -EPERM;
> +		goto out_unlock_kzdev;
> +	}
> +
>  	mutex_lock(&zdev->fmb_lock);
>  	switch (val) {
>  	case 0:
> @@ -163,6 +169,9 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
>  		break;
>  	}
>  	mutex_unlock(&zdev->fmb_lock);
> +
> +out_unlock_kzdev:
> +	mutex_unlock(&zdev->kzdev_lock);
>  	return rc ? rc : count;
>  }
>  

[PATCH v6 4/4] vfio-pci/zdev: Add VFIO FMB device features

[Omar Elghoul]

Introduce new VFIO features for zPCI devices to provide FMB passthrough to
userspace.

Allow the user to enable or disable the FMB using the SET-only feature
VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE. Likewise allow the user to read the
latest FMB using the GET-only feature VFIO_DEVICE_FEATURE_ZPCI_FMB_READ
in the case when the FMB is enabled.

Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
---
 drivers/vfio/pci/vfio_pci_core.c |  4 +++
 drivers/vfio/pci/vfio_pci_priv.h | 18 ++++++++++
 drivers/vfio/pci/vfio_pci_zdev.c | 60 ++++++++++++++++++++++++++++++++
 include/uapi/linux/vfio.h        | 29 +++++++++++++++
 4 files changed, 111 insertions(+)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 6757054e9d87..3c827a77725b 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -1627,6 +1627,10 @@ int vfio_pci_core_ioctl_feature(struct vfio_device *device, u32 flags,
 		return vfio_pci_core_feature_dma_buf(vdev, flags, arg, argsz);
 	case VFIO_DEVICE_FEATURE_ZPCI_ERROR:
 		return vfio_pci_zdev_feature_err(device, flags, arg, argsz);
+	case VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE:
+		return vfio_pci_zdev_feature_fmb_enable(vdev, flags, arg, argsz);
+	case VFIO_DEVICE_FEATURE_ZPCI_FMB_READ:
+		return vfio_pci_zdev_feature_fmb_read(vdev, flags, arg, argsz);
 	default:
 		return -ENOTTY;
 	}
diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
index 4e7162234a2e..e04d7e9d0c30 100644
--- a/drivers/vfio/pci/vfio_pci_priv.h
+++ b/drivers/vfio/pci/vfio_pci_priv.h
@@ -95,6 +95,10 @@ int vfio_pci_zdev_open_device(struct vfio_pci_core_device *vdev);
 void vfio_pci_zdev_close_device(struct vfio_pci_core_device *vdev);
 int vfio_pci_zdev_feature_err(struct vfio_device *device, u32 flags,
 			      void __user *arg, size_t argsz);
+int vfio_pci_zdev_feature_fmb_enable(struct vfio_pci_core_device *vdev, u32 flags,
+				     void __user *arg, size_t argsz);
+int vfio_pci_zdev_feature_fmb_read(struct vfio_pci_core_device *vdev, u32 flags,
+				   void __user *arg, size_t argsz);
 #else
 static inline int vfio_pci_info_zdev_add_caps(struct vfio_pci_core_device *vdev,
 					      struct vfio_info_cap *caps)
@@ -116,6 +120,20 @@ static inline int vfio_pci_zdev_feature_err(struct vfio_device *device,
 {
 	return -ENOTTY;
 }
+
+static inline int vfio_pci_zdev_feature_fmb_enable(struct vfio_pci_core_device *vdev,
+						   u32 flags, void __user *arg,
+						   size_t argsz)
+{
+	return -ENOTTY;
+}
+
+static inline int vfio_pci_zdev_feature_fmb_read(struct vfio_pci_core_device *vdev,
+						 u32 flags, void __user *arg,
+						 size_t argsz)
+{
+	return -ENOTTY;
+}
 #endif
 
 static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
diff --git a/drivers/vfio/pci/vfio_pci_zdev.c b/drivers/vfio/pci/vfio_pci_zdev.c
index f47f36314a1c..b028de8e08a4 100644
--- a/drivers/vfio/pci/vfio_pci_zdev.c
+++ b/drivers/vfio/pci/vfio_pci_zdev.c
@@ -219,3 +219,63 @@ void vfio_pci_zdev_close_device(struct vfio_pci_core_device *vdev)
 	if (zpci_kvm_hook.kvm_unregister)
 		zpci_kvm_hook.kvm_unregister(zdev);
 }
+
+int vfio_pci_zdev_feature_fmb_enable(struct vfio_pci_core_device *vdev, u32 flags,
+				     void __user *arg, size_t argsz)
+{
+	struct zpci_dev *zdev;
+	struct vfio_device_feature_zpci_fmb_enable fmb_enable;
+	int ret;
+
+	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_SET, sizeof(fmb_enable));
+	if (ret != 1)
+		return ret;
+
+	zdev = to_zpci(vdev->pdev);
+	if (!zdev)
+		return -ENODEV;
+
+	if (copy_from_user(&fmb_enable, arg, sizeof(fmb_enable)))
+		return -EFAULT;
+
+	guard(mutex)(&zdev->fmb_lock);
+
+	if (fmb_enable.enabled)
+		return zpci_fmb_reenable_device(zdev);
+	return zpci_fmb_disable_device(zdev);
+}
+
+int vfio_pci_zdev_feature_fmb_read(struct vfio_pci_core_device *vdev, u32 flags,
+				   void __user *arg, size_t argsz)
+{
+	struct zpci_dev *zdev;
+	struct vfio_device_feature_zpci_fmb_read fmb_read;
+	struct zpci_fmb fmb_bounce;
+	int ret;
+
+	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_GET, sizeof(fmb_read));
+	if (ret != 1)
+		return ret;
+
+	zdev = to_zpci(vdev->pdev);
+	if (!zdev)
+		return -ENODEV;
+
+	if (copy_from_user(&fmb_read, arg, sizeof(fmb_read)))
+		return -EFAULT;
+	if (!fmb_read.data)
+		return -EINVAL;
+
+	mutex_lock(&zdev->fmb_lock);
+	if (!zdev->fmb_enabled) {
+		mutex_unlock(&zdev->fmb_lock);
+		return -ENOMSG;
+	}
+
+	memcpy(&fmb_bounce, zdev->fmb, zdev->fmb_length);
+	mutex_unlock(&zdev->fmb_lock);
+
+	if (copy_to_user(u64_to_user_ptr(fmb_read.data), &fmb_bounce, zdev->fmb_length))
+		return -EFAULT;
+	return 0;
+}
diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index e41437fa17ad..a8d501e42ef9 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -1555,6 +1555,35 @@ struct vfio_device_feature_zpci_err {
 
 #define VFIO_DEVICE_FEATURE_ZPCI_ERROR 13
 
+/**
+ * Upon VFIO_DEVICE_FEATURE_SET, enable or disable FMB for the VFIO zPCI device.
+ *
+ * enabled is treated as a bool, so any non-zero value evaluates to true. Nested
+ * enabling resets the FMB counters to zero, and nested disabling fails.
+ *
+ * Returns: 0 on success, -1 and errno set appropriately on error.
+ */
+#define VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE 14
+
+struct vfio_device_feature_zpci_fmb_enable {
+	__u8 enabled;
+};
+
+/**
+ * Upon VFIO_DEVICE_FEATURE_GET, provide FMB passthrough for VFIO zPCI devices.
+ *
+ * The user-provided buffer must be at least fmb_length large, where fmb_length
+ * is reported in VFIO_DEVICE_INFO_CAP_ZPCI_BASE.
+ *
+ * Returns: 0 on success, -1 and errno set appropriately on error. errno==ENOMSG
+ * when the FMB is not enabled.
+ */
+#define VFIO_DEVICE_FEATURE_ZPCI_FMB_READ 15
+
+struct vfio_device_feature_zpci_fmb_read {
+	__aligned_u64 data;
+};
+
 /* -------- API for Type1 VFIO IOMMU -------- */
 
 /**
-- 
2.55.0