[PATCH v12 0/5] vfio-pci/zdev: Improved zPCI Function Measurement Support

[PATCH v12 0/5] 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
=========
v11 -> v12:
* Patch 2/5:
  - Replace the statistics_on bool with a bitfield

* Patch 3/5:
  - Add the fmb_enabled bitfield to struct zpci_dev to track the actual
    FMB enablement
  - Replace the kmem_cache allocator with kzalloc/kfree after learning
    it guarantees alignment for power-of-two sizes

* Patch 5/5:
  - Use fmb_enabled as the FMB liveness check

v10 -> v11:
* Patches 2/5 and 3/5:
  - Swap their ordering within the series
  - Set/unset statistics_on after an enable/disable succeeds instead of
    before

* Patch 5/5:
  - On FMB re-enable, use an intermediate request-off step instead of
    directly touching the device state

v9 -> v10:
* Patches 2/5 and 3/5:
  - Split the old 2/4 into two patches; the new patch 2/5 reuses one FMB
    buffer per device, and 3/5 improves FMB state management
  - Rework FMB management functions to decouple requesting enablement or
    disablement from managing the user's intended state
  - Preserve measurement state in pcibios_enable_device()

v8 -> v9:
* Patch 2/4:
  - Remove the intermediate disable step from zpci_fmb_reenable_device()
    for more uniform semantics
  - Rename fmb_enabled to fmb_requested with the intent of tracking the
    desired status of FMB enablement
  - Make zpci_fmb_enable_device() delegate to zpci_fmb_reenable_device()

* Patch 4/4:
  - Set fmb_requested when the user enables measurement

v7 -> v8:
* Patch 2/4:
  - Replace the fmb_enabled bitfield in struct zpci_dev with a dedicated
    bool to avoid future tearing bugs
  - Rewrite commit message for a more natural flow

* Patch 4/4:
  - Replace mutex_lock/unlock() in vfio_pci_zdev_feature_fmb_read() with
    a scoped_guard()

v6 -> v7:
* Patch 2/4:
  - Don't re-enable FMB if it wasn't already enabled

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 (5):
  s390/pci: Hold fmb_lock when enabling or disabling PCI devices
  s390/pci: Improve function measurement state management
  s390/pci: Improve FMB memory management
  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      |   8 +-
 arch/s390/pci/pci.c              | 121 +++++++++++++++++++++++--------
 arch/s390/pci/pci_debug.c        |  15 +++-
 drivers/vfio/pci/vfio_pci_core.c |   4 +
 drivers/vfio/pci/vfio_pci_priv.h |  18 +++++
 drivers/vfio/pci/vfio_pci_zdev.c |  64 ++++++++++++++++
 include/uapi/linux/vfio.h        |  30 ++++++++
 7 files changed, 224 insertions(+), 36 deletions(-)

-- 
2.56.0

[PATCH v12 1/5] 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>
Reviewed-by: Matthew Rosato <mjrosato@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);
 }
 
-- 
2.56.0

[PATCH v12 2/5] s390/pci: Improve function measurement state management

[Omar Elghoul]

When a zPCI function is disabled and re-enabled, the firmware implicitly
disables function measurement while the kernel still believes it is
enabled, leading to a disconnect between the user-visible state and the
actual device state.

Add the statistics_on field to struct zpci_dev to track the desired FMB
state. Introduce the functions zpci_fmb_request_on()/_off(), which defer
to zpci_fmb_enable()/_disable() to perform the actual firmware change.
This decouples the enablement or disablement step from managing the
user's requested state.

Restore the state in pcibios_enable_device(), pcibios_disable_device(),
and zpci_reenable_device(). New devices start with measurement on by
default, unchanged from before.

Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
---
 arch/s390/include/asm/pci.h |  7 +++--
 arch/s390/pci/pci.c         | 54 +++++++++++++++++++++++++++++++++----
 arch/s390/pci/pci_debug.c   |  4 +--
 3 files changed, 56 insertions(+), 9 deletions(-)

diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
index 88a125b92bdd..dd58c48ac2ed 100644
--- a/arch/s390/include/asm/pci.h
+++ b/arch/s390/include/asm/pci.h
@@ -208,6 +208,7 @@ struct zpci_dev {
 	struct zpci_fmb *fmb;
 	u16		fmb_update;	/* update interval */
 	u16		fmb_length;
+	u8		statistics_on	: 1; /* desired FMB enablement */
 
 	u8		version;
 	enum pci_bus_speed max_bus_speed;
@@ -349,8 +350,10 @@ int zpci_create_parent_msi_domain(struct zpci_bus *zbus);
 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_request_on(struct zpci_dev *zdev);
+int zpci_fmb_request_off(struct zpci_dev *zdev);
+int zpci_fmb_enable(struct zpci_dev *zdev);
+int zpci_fmb_disable(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..f49736386eb7 100644
--- a/arch/s390/pci/pci.c
+++ b/arch/s390/pci/pci.c
@@ -164,8 +164,38 @@ int zpci_unregister_ioat(struct zpci_dev *zdev, u8 dmaas)
 	return cc;
 }
 
+int zpci_fmb_request_on(struct zpci_dev *zdev)
+{
+	int rc;
+
+	lockdep_assert_held(&zdev->fmb_lock);
+
+	rc = zpci_fmb_enable(zdev);
+	if (rc)
+		return rc;
+
+	zdev->statistics_on = 1;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(zpci_fmb_request_on);
+
+int zpci_fmb_request_off(struct zpci_dev *zdev)
+{
+	int rc;
+
+	lockdep_assert_held(&zdev->fmb_lock);
+
+	rc = zpci_fmb_disable(zdev);
+	if (rc)
+		return rc;
+
+	zdev->statistics_on = 0;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(zpci_fmb_request_off);
+
 /* Modify PCI: Set PCI function measurement parameters */
-int zpci_fmb_enable_device(struct zpci_dev *zdev)
+int zpci_fmb_enable(struct zpci_dev *zdev)
 {
 	u64 req = ZPCI_CREATE_REQ(zdev->fh, 0, ZPCI_MOD_FC_SET_MEASURE);
 	struct zpci_iommu_ctrs *ctrs;
@@ -207,7 +237,7 @@ int zpci_fmb_enable_device(struct zpci_dev *zdev)
 }
 
 /* Modify PCI: Disable PCI function measurement */
-int zpci_fmb_disable_device(struct zpci_dev *zdev)
+int zpci_fmb_disable(struct zpci_dev *zdev)
 {
 	u64 req = ZPCI_CREATE_REQ(zdev->fh, 0, ZPCI_MOD_FC_SET_MEASURE);
 	struct zpci_fib fib = {0};
@@ -644,7 +674,8 @@ int pcibios_enable_device(struct pci_dev *pdev, int mask)
 
 	zpci_debug_init_device(zdev, dev_name(&pdev->dev));
 	mutex_lock(&zdev->fmb_lock);
-	zpci_fmb_enable_device(zdev);
+	if (zdev->statistics_on)
+		zpci_fmb_enable(zdev);
 	mutex_unlock(&zdev->fmb_lock);
 
 	return pci_enable_resources(pdev, mask);
@@ -655,7 +686,8 @@ 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);
+	if (zdev->statistics_on)
+		zpci_fmb_disable(zdev);
 	mutex_unlock(&zdev->fmb_lock);
 	zpci_debug_exit_device(zdev);
 }
@@ -737,8 +769,14 @@ 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);
+	if (zdev->statistics_on)
+		zpci_fmb_enable(zdev);
 
 	return rc;
 }
@@ -847,6 +885,12 @@ struct zpci_dev *zpci_create_device(u32 fid, u32 fh, enum zpci_state state)
 		goto error;
 	zdev->state =  state;
 
+	/*
+	 * Measurement is on by default for new functions, this will be honored
+	 * by pcibios_enable_device().
+	 */
+	zdev->statistics_on = 1;
+
 	mutex_init(&zdev->state_lock);
 	mutex_init(&zdev->fmb_lock);
 	mutex_init(&zdev->kzdev_lock);
diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c
index c7ed7bf254b5..06e2e8bb2f82 100644
--- a/arch/s390/pci/pci_debug.c
+++ b/arch/s390/pci/pci_debug.c
@@ -156,10 +156,10 @@ static ssize_t pci_perf_seq_write(struct file *file, const char __user *ubuf,
 	mutex_lock(&zdev->fmb_lock);
 	switch (val) {
 	case 0:
-		rc = zpci_fmb_disable_device(zdev);
+		rc = zpci_fmb_request_off(zdev);
 		break;
 	case 1:
-		rc = zpci_fmb_enable_device(zdev);
+		rc = zpci_fmb_request_on(zdev);
 		break;
 	}
 	mutex_unlock(&zdev->fmb_lock);
-- 
2.56.0

[PATCH v12 3/5] s390/pci: Improve FMB memory management

[Omar Elghoul]

Refactor the FMB memory management code to use kzalloc() instead of a
dedicated kmem_cache. kzalloc() already guarantees adequate alignment
for power-of-two sizes, rendering the kmem_cache approach unnecessary.
This simplifies both the initialization and cleanup procedures.

Allocate the FMB once upon first enable and keep it for the lifetime of
the zdev instead of freeing it on every disable. Defer freeing the
buffer until device teardown in zpci_release_device(). Add the field
fmb_enabled to struct zpci_dev as the new FMB liveness bool, since a
non-null FMB pointer no longer implies FMB enablement.

Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
---
 arch/s390/include/asm/pci.h |  1 +
 arch/s390/pci/pci.c         | 59 +++++++++++++++++++++----------------
 arch/s390/pci/pci_debug.c   |  2 +-
 3 files changed, 35 insertions(+), 27 deletions(-)

diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
index dd58c48ac2ed..6d3c1b8ac75b 100644
--- a/arch/s390/include/asm/pci.h
+++ b/arch/s390/include/asm/pci.h
@@ -209,6 +209,7 @@ struct zpci_dev {
 	u16		fmb_update;	/* update interval */
 	u16		fmb_length;
 	u8		statistics_on	: 1; /* desired FMB enablement */
+	u8		fmb_enabled	: 1; /* actual FMB enablement */
 
 	u8		version;
 	enum pci_bus_speed max_bus_speed;
diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
index f49736386eb7..f066a7ffc35c 100644
--- a/arch/s390/pci/pci.c
+++ b/arch/s390/pci/pci.c
@@ -62,8 +62,6 @@ EXPORT_SYMBOL_GPL(zpci_iomap_start);
 
 DEFINE_STATIC_KEY_FALSE(have_mio);
 
-static struct kmem_cache *zdev_fmb_cache;
-
 /* AEN structures that must be preserved over KVM module re-insertion */
 union zpci_sic_iib *zpci_aipb;
 EXPORT_SYMBOL_GPL(zpci_aipb);
@@ -205,13 +203,17 @@ int zpci_fmb_enable(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 = kzalloc_obj(*zdev->fmb, GFP_KERNEL);
+		if (!zdev->fmb)
+			return -ENOMEM;
+	} else {
+		/* reuse the same FMB buffer for as long as the zdev lives */
+		memset(zdev->fmb, 0, sizeof(*zdev->fmb));
+	}
 
 	/* reset software counters */
 	spin_lock_irqsave(&zdev->dom_lock, flags);
@@ -229,11 +231,12 @@ int zpci_fmb_enable(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 */
@@ -245,7 +248,7 @@ int zpci_fmb_disable(struct zpci_dev *zdev)
 
 	lockdep_assert_held(&zdev->fmb_lock);
 
-	if (!zdev->fmb)
+	if (!zdev->fmb_enabled)
 		return -EINVAL;
 
 	fib.gd = zdev->gisa;
@@ -255,11 +258,11 @@ int zpci_fmb_disable(struct zpci_dev *zdev)
 	if (cc == 3) /* Function already gone. */
 		cc = 0;
 
-	if (!cc) {
-		kmem_cache_free(zdev_fmb_cache, zdev->fmb);
-		zdev->fmb = NULL;
-	}
-	return cc ? -EIO : 0;
+	if (cc)
+		return -EIO;
+
+	zdev->fmb_enabled = 0;
+	return 0;
 }
 
 static int zpci_cfg_load(struct zpci_dev *zdev, int offset, u32 *val, u8 len)
@@ -802,6 +805,12 @@ int zpci_disable_device(struct zpci_dev *zdev)
 	} else {
 		rc = -EIO;
 	}
+
+	if (!rc) {
+		guard(mutex)(&zdev->fmb_lock);
+		zdev->fmb_enabled = 0;
+	}
+
 	return rc;
 }
 EXPORT_SYMBOL_GPL(zpci_disable_device);
@@ -1047,6 +1056,12 @@ void zpci_release_device(struct kref *kref)
 	if (zdev->has_resources)
 		zpci_cleanup_bus_resources(zdev);
 
+	if (zdev->fmb) {
+		zdev->fmb_enabled = 0;
+		kfree(zdev->fmb);
+		zdev->fmb = NULL;
+	}
+
 	zpci_bus_device_unregister(zdev);
 	zpci_destroy_iommu(zdev);
 	zpci_dbg(3, "rem fid:%x\n", zdev->fid);
@@ -1122,11 +1137,6 @@ static int zpci_mem_init(void)
 	BUILD_BUG_ON((CONFIG_ILLEGAL_POINTER_VALUE + 0x10000 > ZPCI_IOMAP_ADDR_BASE) &&
 		     (CONFIG_ILLEGAL_POINTER_VALUE <= ZPCI_IOMAP_ADDR_MAX));
 
-	zdev_fmb_cache = kmem_cache_create("PCI_FMB_cache", sizeof(struct zpci_fmb),
-					   __alignof__(struct zpci_fmb), 0, NULL);
-	if (!zdev_fmb_cache)
-		goto error_fmb;
-
 	zpci_iomap_start = kzalloc_objs(*zpci_iomap_start, ZPCI_IOMAP_ENTRIES);
 	if (!zpci_iomap_start)
 		goto error_iomap;
@@ -1143,8 +1153,6 @@ static int zpci_mem_init(void)
 error_iomap_bitmap:
 	kfree(zpci_iomap_start);
 error_iomap:
-	kmem_cache_destroy(zdev_fmb_cache);
-error_fmb:
 	return -ENOMEM;
 }
 
@@ -1152,7 +1160,6 @@ static void zpci_mem_exit(void)
 {
 	kfree(zpci_iomap_bitmap);
 	kfree(zpci_iomap_start);
-	kmem_cache_destroy(zdev_fmb_cache);
 }
 
 static unsigned int s390_pci_probe __initdata = 1;
diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c
index 06e2e8bb2f82..454121bc274c 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;
-- 
2.56.0

[PATCH v12 4/5] 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>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@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 454121bc274c..02da077b5170 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;
 }
 
-- 
2.56.0

[PATCH v12 5/5] 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>
Acked-by: Alex Williamson <alex@shazbot.org>
---
 drivers/vfio/pci/vfio_pci_core.c |  4 ++
 drivers/vfio/pci/vfio_pci_priv.h | 18 +++++++++
 drivers/vfio/pci/vfio_pci_zdev.c | 64 ++++++++++++++++++++++++++++++++
 include/uapi/linux/vfio.h        | 30 +++++++++++++++
 4 files changed, 116 insertions(+)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index fcbb2da860e2..f2d5b6857925 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -1614,6 +1614,10 @@ 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..0584fbaf8755 100644
--- a/drivers/vfio/pci/vfio_pci_zdev.c
+++ b/drivers/vfio/pci/vfio_pci_zdev.c
@@ -219,3 +219,67 @@ 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) {
+		ret = zpci_fmb_request_off(zdev);
+		if (ret && ret != -EINVAL)
+			return ret;
+
+		return zpci_fmb_request_on(zdev);
+	}
+
+	return zpci_fmb_request_off(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;
+
+	scoped_guard(mutex, &zdev->fmb_lock) {
+		if (!zdev->fmb_enabled)
+			return -ENOMSG;
+		memcpy(&fmb_bounce, zdev->fmb, zdev->fmb_length);
+	}
+
+	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..7e9adb61c5e1 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -1555,6 +1555,36 @@ 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 disables and re-enables the FMB, while nested disabling fails with
+ * EINVAL.
+ *
+ * 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. If measurement is not enabled,
+ * fail with ENOMSG.
+ *
+ * Returns: 0 on success, -1 and errno set appropriately on error.
+ */
+#define VFIO_DEVICE_FEATURE_ZPCI_FMB_READ 15
+
+struct vfio_device_feature_zpci_fmb_read {
+	__aligned_u64 data;
+};
+
 /* -------- API for Type1 VFIO IOMMU -------- */
 
 /**
-- 
2.56.0

Re: [PATCH v12 5/5] vfio-pci/zdev: Add VFIO FMB device features

[Niklas Schnelle]

On Tue, 2026-11-03 at 10:01 -0500, Omar Elghoul wrote:
> 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>
> Acked-by: Alex Williamson <alex@shazbot.org>
> ---
>  drivers/vfio/pci/vfio_pci_core.c |  4 ++
>  drivers/vfio/pci/vfio_pci_priv.h | 18 +++++++++
>  drivers/vfio/pci/vfio_pci_zdev.c | 64 ++++++++++++++++++++++++++++++++
>  include/uapi/linux/vfio.h        | 30 +++++++++++++++
>  4 files changed, 116 insertions(+)
> 
--- snip ---
> +
> +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;
> +
> +	scoped_guard(mutex, &zdev->fmb_lock) {
> +		if (!zdev->fmb_enabled)
> +			return -ENOMSG;

For those who were also wondering what would happen if the zdev has an
FMB length too large for this kernel's version of struct zpci_fmb. In
that case zpci_fmb_enable() fails the FMB enablement leaving zdev-
>fmb_enabled false and then the check above also covers the memcpy()
here even if user-space ignores that the FMB is not enabled.

> +		memcpy(&fmb_bounce, zdev->fmb, zdev->fmb_length);
> +	}
> +
> +	if (copy_to_user(u64_to_user_ptr(fmb_read.data), &fmb_bounce, zdev->fmb_length))
> +		return -EFAULT;
> +
> +	return 0;
> +}

Looks good, looking forward to having function measurement in KVM
guests.

Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>

Re: [PATCH v12 5/5] vfio-pci/zdev: Add VFIO FMB device features

[Matthew Rosato]

On 11/3/26 4:08 PM, Niklas Schnelle wrote:
> On Tue, 2026-11-03 at 10:01 -0500, Omar Elghoul wrote:
>> 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>
>> Acked-by: Alex Williamson <alex@shazbot.org>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>