[PATCH v21 0/4] [PCI] Error recovery for vfio-pci devices on s390x

[PATCH v21 0/4] [PCI] Error recovery for vfio-pci devices on s390x

[Farhan Ali]

Hi Bjorn,

This patch set includes only the PCI patches of the original series for
error recovery for vfio-pci devices on s390x [1]. Breaking up the patch
series into PCI and VFIO only patches to make merging easier based on
discussion with Alex [2].

Thanks
Farhan

[1] https://lore.kernel.org/all/20260520171113.1111-1-alifm@linux.ibm.com/
[2] https://lore.kernel.org/all/20260602163344.1eda12d2@shazbot.org/

ChangeLog
---------
v20 https://lore.kernel.org/all/20260622171840.1618-1-alifm@linux.ibm.com/
v20 -> v21
  - Amend commit message to include Fixes tag and cc stable (patch 4).
  - Rebase on 7.2-rc1.

v19 https://lore.kernel.org/all/20260615183524.2880-1-alifm@linux.ibm.com/
v19 -> v20
  - Unconditionally enable Memory bit while restoring MSI-X (patch 4).
  Fixes an issue found with sashiko. 

v18 https://lore.kernel.org/all/20260603181647.2215-1-alifm@linux.ibm.com/
v18 -> v19
  - Move config space accessible check to pcie_flr() function (based on
  discussion of Sashiko review)

  - Fix a gap in MSI-X restoration (patch 4).

  - Rebase on 7.1-rc7

v17 -> v18
  - Rebase on 7.1-rc6.

Farhan Ali (4):
  PCI: Allow per function PCI slots to fix slot reset on s390
  PCI: Avoid saving config space state if inaccessible
  PCI: Fail FLR when config space is inaccessible
  PCI/MSI: Enable memory decoding before restoring MSI-X messages

 drivers/pci/hotplug/rpaphp_slot.c |  2 +-
 drivers/pci/msi/msi.c             | 10 ++++++++++
 drivers/pci/pci.c                 | 32 ++++++++++++++++++++++++++++--
 drivers/pci/slot.c                | 33 +++++++++++++++++++++++--------
 include/linux/pci.h               |  8 ++++++--
 5 files changed, 72 insertions(+), 13 deletions(-)

-- 
2.43.0

[PATCH v21 1/4] PCI: Allow per function PCI slots to fix slot reset on s390

[Farhan Ali]

On s390 systems, which use a machine level hypervisor, PCI devices are
always accessed through a form of PCI pass-through which fundamentally
operates on a per PCI function granularity. This is also reflected in the
s390 PCI hotplug driver which creates hotplug slots for individual PCI
functions. Its reset_slot() function, which is a wrapper for
zpci_hot_reset_device(), thus also resets individual functions.

Currently, the kernel's PCI_SLOT() macro assigns the same pci_slot object
to multifunction devices. This approach worked fine on s390 systems that
only exposed virtual functions as individual PCI domains to the operating
system.  Since commit 44510d6fa0c0 ("s390/pci: Handling multifunctions")
s390 supports exposing the topology of multifunction PCI devices by
grouping them in a shared PCI domain. This creates a problem when resetting
a function through the hotplug driver's slot_reset() interface.

When attempting to reset a function through the hotplug driver, the shared
slot assignment causes the wrong function to be reset instead of the
intended one. It also leaks memory as we do create a pci_slot object for
the function, but don't correctly free it in pci_slot_release().

Add a flag for struct pci_slot to allow per function PCI slots for
functions managed through a hypervisor, which exposes individual PCI
functions while retaining the topology. Since we can use all 8 bits
for slot 'number' (for ARI devices), change slot 'number' u16 to
account for special values -1 and PCI_SLOT_ALL_DEVICES.

Fixes: 44510d6fa0c0 ("s390/pci: Handling multifunctions")
Cc: stable@vger.kernel.org
Suggested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
---
 drivers/pci/hotplug/rpaphp_slot.c |  2 +-
 drivers/pci/pci.c                 |  5 +++--
 drivers/pci/slot.c                | 33 +++++++++++++++++++++++--------
 include/linux/pci.h               |  8 ++++++--
 4 files changed, 35 insertions(+), 13 deletions(-)

diff --git a/drivers/pci/hotplug/rpaphp_slot.c b/drivers/pci/hotplug/rpaphp_slot.c
index 67362e5b9971..92eabf5f61b9 100644
--- a/drivers/pci/hotplug/rpaphp_slot.c
+++ b/drivers/pci/hotplug/rpaphp_slot.c
@@ -84,7 +84,7 @@ int rpaphp_register_slot(struct slot *slot)
 	struct hotplug_slot *php_slot = &slot->hotplug_slot;
 	u32 my_index;
 	int retval;
-	int slotno = -1;
+	int slotno = PCI_SLOT_PLACEHOLDER;
 
 	dbg("%s registering slot:path[%pOF] index[%x], name[%s] pdomain[%x] type[%d]\n",
 		__func__, slot->dn, slot->index, slot->name,
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 77b17b13ee61..350bae907ebf 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4897,8 +4897,9 @@ static int pci_reset_hotplug_slot(struct hotplug_slot *hotplug, bool probe)
 
 static int pci_dev_reset_slot_function(struct pci_dev *dev, bool probe)
 {
-	if (dev->multifunction || dev->subordinate || !dev->slot ||
-	    dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET)
+	if (dev->subordinate || !dev->slot ||
+	    dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET ||
+	    (dev->multifunction && !dev->slot->per_func_slot))
 		return -ENOTTY;
 
 	return pci_reset_hotplug_slot(dev->slot->hotplug, probe);
diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
index 6d5cd37bfb1e..894d6213ed30 100644
--- a/drivers/pci/slot.c
+++ b/drivers/pci/slot.c
@@ -37,7 +37,7 @@ static const struct sysfs_ops pci_slot_sysfs_ops = {
 
 static ssize_t address_read_file(struct pci_slot *slot, char *buf)
 {
-	if (slot->number == 0xff)
+	if (slot->number == (u16)PCI_SLOT_PLACEHOLDER)
 		return sysfs_emit(buf, "%04x:%02x\n",
 				  pci_domain_nr(slot->bus),
 				  slot->bus->number);
@@ -72,6 +72,23 @@ static ssize_t cur_speed_read_file(struct pci_slot *slot, char *buf)
 	return bus_speed_read(slot->bus->cur_bus_speed, buf);
 }
 
+static bool pci_dev_matches_slot(struct pci_dev *dev, struct pci_slot *slot)
+{
+	if (slot->per_func_slot)
+		return dev->devfn == slot->number;
+
+	return slot->number == PCI_SLOT_ALL_DEVICES ||
+		PCI_SLOT(dev->devfn) == slot->number;
+}
+
+static bool pci_slot_enabled_per_func(void)
+{
+	if (IS_ENABLED(CONFIG_S390))
+		return true;
+
+	return false;
+}
+
 static void pci_slot_release(struct kobject *kobj)
 {
 	struct pci_dev *dev;
@@ -82,8 +99,7 @@ static void pci_slot_release(struct kobject *kobj)
 
 	down_read(&pci_bus_sem);
 	list_for_each_entry(dev, &slot->bus->devices, bus_list)
-		if (slot->number == PCI_SLOT_ALL_DEVICES ||
-		    PCI_SLOT(dev->devfn) == slot->number)
+		if (pci_dev_matches_slot(dev, slot))
 			dev->slot = NULL;
 	up_read(&pci_bus_sem);
 
@@ -187,8 +203,7 @@ void pci_dev_assign_slot(struct pci_dev *dev)
 
 	mutex_lock(&pci_slot_mutex);
 	list_for_each_entry(slot, &dev->bus->slots, list)
-		if (slot->number == PCI_SLOT_ALL_DEVICES ||
-		    PCI_SLOT(dev->devfn) == slot->number)
+		if (pci_dev_matches_slot(dev, slot))
 			dev->slot = slot;
 	mutex_unlock(&pci_slot_mutex);
 }
@@ -267,7 +282,7 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
 
 	mutex_lock(&pci_slot_mutex);
 
-	if (slot_nr == -1)
+	if (slot_nr == PCI_SLOT_PLACEHOLDER)
 		goto placeholder;
 
 	/*
@@ -298,6 +313,9 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
 	slot->bus = pci_bus_get(parent);
 	slot->number = slot_nr;
 
+	if (pci_slot_enabled_per_func())
+		slot->per_func_slot = 1;
+
 	slot->kobj.kset = pci_slots_kset;
 
 	slot_name = make_slot_name(name);
@@ -318,8 +336,7 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
 
 	down_read(&pci_bus_sem);
 	list_for_each_entry(dev, &parent->devices, bus_list)
-		if (slot_nr == PCI_SLOT_ALL_DEVICES ||
-		    PCI_SLOT(dev->devfn) == slot_nr)
+		if (pci_dev_matches_slot(dev, slot))
 			dev->slot = slot;
 	up_read(&pci_bus_sem);
 
diff --git a/include/linux/pci.h b/include/linux/pci.h
index ebb5b9d76360..b6e20616e17f 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -79,14 +79,18 @@
  * and, if ARI Forwarding is enabled, functions may appear to be on multiple
  * devices.
  */
-#define PCI_SLOT_ALL_DEVICES	0xfe
+#define PCI_SLOT_ALL_DEVICES	0xfeff
+
+/* Used to identify a slot as a placeholder */
+#define PCI_SLOT_PLACEHOLDER	-1
 
 /* pci_slot represents a physical slot */
 struct pci_slot {
 	struct pci_bus		*bus;		/* Bus this slot is on */
 	struct list_head	list;		/* Node in list of slots */
 	struct hotplug_slot	*hotplug;	/* Hotplug info (move here) */
-	unsigned char		number;		/* Device nr, or PCI_SLOT_ALL_DEVICES */
+	u16			number;		/* Device nr, or PCI_SLOT_ALL_DEVICES */
+	unsigned int		per_func_slot:1; /* Allow per function slot */
 	struct kobject		kobj;
 };
 
-- 
2.43.0

[PATCH v21 2/4] PCI: Avoid saving config space state if inaccessible

[Farhan Ali]

The current reset process saves the device's config space state before
reset and restores it afterward. However errors may occur unexpectedly and
it may then be impossible to save config space because the device may be
inaccessible (e.g. DPC). This results in saving invalid values that get
written back to the device during state restoration.

With a reset we want to recover/restore the device into a functional state.
So avoid saving the state of the config space when the device config space
is inaccessible.

Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
---
 drivers/pci/pci.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 350bae907ebf..e8d7de77241a 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -713,6 +713,27 @@ u16 pci_find_dvsec_capability(struct pci_dev *dev, u16 vendor, u16 dvsec)
 }
 EXPORT_SYMBOL_GPL(pci_find_dvsec_capability);
 
+static bool pci_dev_config_accessible(struct pci_dev *dev, char *msg)
+{
+	u32 val;
+
+	/*
+	 * If device's config space is inaccessible it can return ~0 for
+	 * any reads. Since VFs can also return ~0 for Device and Vendor ID
+	 * check Command and Status registers. Note that this is racy
+	 * because the device may become inaccessible partway through
+	 * next access.
+	 */
+	pci_read_config_dword(dev, PCI_COMMAND, &val);
+	if (PCI_POSSIBLE_ERROR(val)) {
+		pci_warn(dev, "Device config space inaccessible; unable to %s\n",
+				msg);
+		return false;
+	}
+
+	return true;
+}
+
 /**
  * pci_find_parent_resource - return resource region of parent bus of given
  *			      region
@@ -5059,6 +5080,9 @@ static void pci_dev_save_and_disable(struct pci_dev *dev)
 	 */
 	pci_set_power_state(dev, PCI_D0);
 
+	if (!pci_dev_config_accessible(dev, "save state"))
+		return;
+
 	pci_save_state(dev);
 	/*
 	 * Disable the device by clearing the Command register, except for
-- 
2.43.0

[PATCH v21 3/4] PCI: Fail FLR when config space is inaccessible

[Farhan Ali]

If a device is in an error state, then it's config space may not be
accssible. Add additional check to validate if a device's config space is
accessible before doing an FLR reset.

Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
---
 drivers/pci/pci.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index e8d7de77241a..9a9d021301c4 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4384,6 +4384,9 @@ int pcie_flr(struct pci_dev *dev)
 {
 	int ret;
 
+	if (!pci_dev_config_accessible(dev, "FLR"))
+		return -ENOTTY;
+
 	if (!pci_wait_for_pending_transaction(dev))
 		pci_err(dev, "timed out waiting for pending transaction; performing function level reset anyway\n");
 
-- 
2.43.0

[PATCH v21 4/4] PCI/MSI: Enable memory decoding before restoring MSI-X messages

[Farhan Ali]

The current MSI-X restoration path assumes the Command register Memory bit
is enabled when writing MSI-X messages. But it's possible the last saved
and restored state of a device may not have the Memory bit enabled, even if
a device driver later enables Memory bit and MSI-X. Attempting to access
Memory space without Memory bit enabled can lead to Unsupported Request
(UR) from the device. Fix this by enabling Memory bit and restore it
afterwards.

Fixes: 41017f0cac92 ("[PATCH] PCI: MSI(X) save/restore for suspend/resume")
Cc: stable@vger.kernel.org
Reviewed-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
---
 drivers/pci/msi/msi.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/pci/msi/msi.c b/drivers/pci/msi/msi.c
index 209373c92e9e..79c7e84d314b 100644
--- a/drivers/pci/msi/msi.c
+++ b/drivers/pci/msi/msi.c
@@ -870,6 +870,7 @@ void __pci_restore_msix_state(struct pci_dev *dev)
 {
 	struct msi_desc *entry;
 	bool write_msg;
+	u16 cmd;
 
 	if (!dev->msix_enabled)
 		return;
@@ -879,6 +880,14 @@ void __pci_restore_msix_state(struct pci_dev *dev)
 	pci_msix_clear_and_set_ctrl(dev, 0,
 				PCI_MSIX_FLAGS_ENABLE | PCI_MSIX_FLAGS_MASKALL);
 
+	/*
+	 * The restored device state may not have Memory decoding enabled
+	 * in the Command register. Since the MSI-X was enabled for the
+	 * device, enable Memory decoding before restoring MSI-X.
+	 */
+	pci_read_config_word(dev, PCI_COMMAND, &cmd);
+	pci_write_config_word(dev, PCI_COMMAND, cmd | PCI_COMMAND_MEMORY);
+
 	write_msg = arch_restore_msi_irqs(dev);
 
 	scoped_guard (msi_descs_lock, &dev->dev) {
@@ -889,6 +898,7 @@ void __pci_restore_msix_state(struct pci_dev *dev)
 		}
 	}
 
+	pci_write_config_word(dev, PCI_COMMAND, cmd);
 	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_MASKALL, 0);
 }
 
-- 
2.43.0

Re: [PATCH v21 0/4] [PCI] Error recovery for vfio-pci devices on s390x

[Farhan Ali]

Polite ping for this series.

Thanks

Farhan

On 6/30/2026 9:48 AM, Farhan Ali wrote:
> Hi Bjorn,
>
> This patch set includes only the PCI patches of the original series for
> error recovery for vfio-pci devices on s390x [1]. Breaking up the patch
> series into PCI and VFIO only patches to make merging easier based on
> discussion with Alex [2].
>
> Thanks
> Farhan
>
> [1] https://lore.kernel.org/all/20260520171113.1111-1-alifm@linux.ibm.com/
> [2] https://lore.kernel.org/all/20260602163344.1eda12d2@shazbot.org/
>
> ChangeLog
> ---------
> v20 https://lore.kernel.org/all/20260622171840.1618-1-alifm@linux.ibm.com/
> v20 -> v21
>    - Amend commit message to include Fixes tag and cc stable (patch 4).
>    - Rebase on 7.2-rc1.
>
> v19 https://lore.kernel.org/all/20260615183524.2880-1-alifm@linux.ibm.com/
> v19 -> v20
>    - Unconditionally enable Memory bit while restoring MSI-X (patch 4).
>    Fixes an issue found with sashiko.
>
> v18 https://lore.kernel.org/all/20260603181647.2215-1-alifm@linux.ibm.com/
> v18 -> v19
>    - Move config space accessible check to pcie_flr() function (based on
>    discussion of Sashiko review)
>
>    - Fix a gap in MSI-X restoration (patch 4).
>
>    - Rebase on 7.1-rc7
>
> v17 -> v18
>    - Rebase on 7.1-rc6.
>
> Farhan Ali (4):
>    PCI: Allow per function PCI slots to fix slot reset on s390
>    PCI: Avoid saving config space state if inaccessible
>    PCI: Fail FLR when config space is inaccessible
>    PCI/MSI: Enable memory decoding before restoring MSI-X messages
>
>   drivers/pci/hotplug/rpaphp_slot.c |  2 +-
>   drivers/pci/msi/msi.c             | 10 ++++++++++
>   drivers/pci/pci.c                 | 32 ++++++++++++++++++++++++++++--
>   drivers/pci/slot.c                | 33 +++++++++++++++++++++++--------
>   include/linux/pci.h               |  8 ++++++--
>   5 files changed, 72 insertions(+), 13 deletions(-)
>

Re: [PATCH v21 1/4] PCI: Allow per function PCI slots to fix slot reset on s390

[Bjorn Helgaas]

On Tue, Jun 30, 2026 at 09:48:04AM -0700, Farhan Ali wrote:
> On s390 systems, which use a machine level hypervisor, PCI devices are
> always accessed through a form of PCI pass-through which fundamentally
> operates on a per PCI function granularity. This is also reflected in the
> s390 PCI hotplug driver which creates hotplug slots for individual PCI
> functions. Its reset_slot() function, which is a wrapper for
> zpci_hot_reset_device(), thus also resets individual functions.
> 
> Currently, the kernel's PCI_SLOT() macro assigns the same pci_slot object
> to multifunction devices.

PCI_SLOT() doesn't assign pci_slot objects; I guess they're assigned
by some code that *uses* PCI_SLOT().  Since this says "currently," I
assume you're changing that code, so we should mention where it is to
help readers out.

I see some Sashiko comments; those also need to be addressed or
explained away.

> This approach worked fine on s390 systems that
> only exposed virtual functions as individual PCI domains to the operating
> system.  Since commit 44510d6fa0c0 ("s390/pci: Handling multifunctions")
> s390 supports exposing the topology of multifunction PCI devices by
> grouping them in a shared PCI domain. This creates a problem when resetting
> a function through the hotplug driver's slot_reset() interface.
> 
> When attempting to reset a function through the hotplug driver, the shared
> slot assignment causes the wrong function to be reset instead of the
> intended one. It also leaks memory as we do create a pci_slot object for
> the function, but don't correctly free it in pci_slot_release().
> 
> Add a flag for struct pci_slot to allow per function PCI slots for
> functions managed through a hypervisor, which exposes individual PCI
> functions while retaining the topology. Since we can use all 8 bits
> for slot 'number' (for ARI devices), change slot 'number' u16 to
> account for special values -1 and PCI_SLOT_ALL_DEVICES.
> 
> Fixes: 44510d6fa0c0 ("s390/pci: Handling multifunctions")
> Cc: stable@vger.kernel.org
> Suggested-by: Niklas Schnelle <schnelle@linux.ibm.com>
> Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
> ---
>  drivers/pci/hotplug/rpaphp_slot.c |  2 +-
>  drivers/pci/pci.c                 |  5 +++--
>  drivers/pci/slot.c                | 33 +++++++++++++++++++++++--------
>  include/linux/pci.h               |  8 ++++++--
>  4 files changed, 35 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/pci/hotplug/rpaphp_slot.c b/drivers/pci/hotplug/rpaphp_slot.c
> index 67362e5b9971..92eabf5f61b9 100644
> --- a/drivers/pci/hotplug/rpaphp_slot.c
> +++ b/drivers/pci/hotplug/rpaphp_slot.c
> @@ -84,7 +84,7 @@ int rpaphp_register_slot(struct slot *slot)
>  	struct hotplug_slot *php_slot = &slot->hotplug_slot;
>  	u32 my_index;
>  	int retval;
> -	int slotno = -1;
> +	int slotno = PCI_SLOT_PLACEHOLDER;
>  
>  	dbg("%s registering slot:path[%pOF] index[%x], name[%s] pdomain[%x] type[%d]\n",
>  		__func__, slot->dn, slot->index, slot->name,
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 77b17b13ee61..350bae907ebf 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -4897,8 +4897,9 @@ static int pci_reset_hotplug_slot(struct hotplug_slot *hotplug, bool probe)
>  
>  static int pci_dev_reset_slot_function(struct pci_dev *dev, bool probe)
>  {
> -	if (dev->multifunction || dev->subordinate || !dev->slot ||
> -	    dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET)
> +	if (dev->subordinate || !dev->slot ||
> +	    dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET ||
> +	    (dev->multifunction && !dev->slot->per_func_slot))
>  		return -ENOTTY;
>  
>  	return pci_reset_hotplug_slot(dev->slot->hotplug, probe);
> diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
> index 6d5cd37bfb1e..894d6213ed30 100644
> --- a/drivers/pci/slot.c
> +++ b/drivers/pci/slot.c
> @@ -37,7 +37,7 @@ static const struct sysfs_ops pci_slot_sysfs_ops = {
>  
>  static ssize_t address_read_file(struct pci_slot *slot, char *buf)
>  {
> -	if (slot->number == 0xff)
> +	if (slot->number == (u16)PCI_SLOT_PLACEHOLDER)
>  		return sysfs_emit(buf, "%04x:%02x\n",
>  				  pci_domain_nr(slot->bus),
>  				  slot->bus->number);
> @@ -72,6 +72,23 @@ static ssize_t cur_speed_read_file(struct pci_slot *slot, char *buf)
>  	return bus_speed_read(slot->bus->cur_bus_speed, buf);
>  }
>  
> +static bool pci_dev_matches_slot(struct pci_dev *dev, struct pci_slot *slot)
> +{
> +	if (slot->per_func_slot)
> +		return dev->devfn == slot->number;
> +
> +	return slot->number == PCI_SLOT_ALL_DEVICES ||
> +		PCI_SLOT(dev->devfn) == slot->number;
> +}
> +
> +static bool pci_slot_enabled_per_func(void)
> +{
> +	if (IS_ENABLED(CONFIG_S390))
> +		return true;
> +
> +	return false;
> +}
> +
>  static void pci_slot_release(struct kobject *kobj)
>  {
>  	struct pci_dev *dev;
> @@ -82,8 +99,7 @@ static void pci_slot_release(struct kobject *kobj)
>  
>  	down_read(&pci_bus_sem);
>  	list_for_each_entry(dev, &slot->bus->devices, bus_list)
> -		if (slot->number == PCI_SLOT_ALL_DEVICES ||
> -		    PCI_SLOT(dev->devfn) == slot->number)
> +		if (pci_dev_matches_slot(dev, slot))
>  			dev->slot = NULL;
>  	up_read(&pci_bus_sem);
>  
> @@ -187,8 +203,7 @@ void pci_dev_assign_slot(struct pci_dev *dev)
>  
>  	mutex_lock(&pci_slot_mutex);
>  	list_for_each_entry(slot, &dev->bus->slots, list)
> -		if (slot->number == PCI_SLOT_ALL_DEVICES ||
> -		    PCI_SLOT(dev->devfn) == slot->number)
> +		if (pci_dev_matches_slot(dev, slot))
>  			dev->slot = slot;
>  	mutex_unlock(&pci_slot_mutex);
>  }
> @@ -267,7 +282,7 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>  
>  	mutex_lock(&pci_slot_mutex);
>  
> -	if (slot_nr == -1)
> +	if (slot_nr == PCI_SLOT_PLACEHOLDER)
>  		goto placeholder;
>  
>  	/*
> @@ -298,6 +313,9 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>  	slot->bus = pci_bus_get(parent);
>  	slot->number = slot_nr;
>  
> +	if (pci_slot_enabled_per_func())
> +		slot->per_func_slot = 1;
> +
>  	slot->kobj.kset = pci_slots_kset;
>  
>  	slot_name = make_slot_name(name);
> @@ -318,8 +336,7 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>  
>  	down_read(&pci_bus_sem);
>  	list_for_each_entry(dev, &parent->devices, bus_list)
> -		if (slot_nr == PCI_SLOT_ALL_DEVICES ||
> -		    PCI_SLOT(dev->devfn) == slot_nr)
> +		if (pci_dev_matches_slot(dev, slot))
>  			dev->slot = slot;
>  	up_read(&pci_bus_sem);
>  
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index ebb5b9d76360..b6e20616e17f 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -79,14 +79,18 @@
>   * and, if ARI Forwarding is enabled, functions may appear to be on multiple
>   * devices.
>   */
> -#define PCI_SLOT_ALL_DEVICES	0xfe
> +#define PCI_SLOT_ALL_DEVICES	0xfeff
> +
> +/* Used to identify a slot as a placeholder */
> +#define PCI_SLOT_PLACEHOLDER	-1
>  
>  /* pci_slot represents a physical slot */
>  struct pci_slot {
>  	struct pci_bus		*bus;		/* Bus this slot is on */
>  	struct list_head	list;		/* Node in list of slots */
>  	struct hotplug_slot	*hotplug;	/* Hotplug info (move here) */
> -	unsigned char		number;		/* Device nr, or PCI_SLOT_ALL_DEVICES */
> +	u16			number;		/* Device nr, or PCI_SLOT_ALL_DEVICES */
> +	unsigned int		per_func_slot:1; /* Allow per function slot */
>  	struct kobject		kobj;
>  };
>  
> -- 
> 2.43.0
> 

Re: [PATCH v21 1/4] PCI: Allow per function PCI slots to fix slot reset on s390

[Farhan Ali]

On 7/15/2026 4:34 PM, Bjorn Helgaas wrote:
>> On s390 systems, which use a machine level hypervisor, PCI devices are
>> always accessed through a form of PCI pass-through which fundamentally
>> operates on a per PCI function granularity. This is also reflected in the
>> s390 PCI hotplug driver which creates hotplug slots for individual PCI
>> functions. Its reset_slot() function, which is a wrapper for
>> zpci_hot_reset_device(), thus also resets individual functions.
>>
>> Currently, the kernel's PCI_SLOT() macro assigns the same pci_slot object
>> to multifunction devices.
> PCI_SLOT() doesn't assign pci_slot objects; I guess they're assigned
> by some code that*uses* PCI_SLOT().  Since this says "currently," I
> assume you're changing that code, so we should mention where it is to
> help readers out.

Thanks for your response! I can re-word the commit message, how about 
something like this:

Currently, the pci_create_slot() assigns the same pci_slot object to 
multifunction devices.


>
> I see some Sashiko comments; those also need to be addressed or
> explained away.

Regarding Sashiko's comments for this patch, it mentions 2 issues:

New issues: - [High] Unconditional enablement of `per_func_slot` on S390 
breaks standard PCI hotplug (e.g., pciehp, shpchp) slot matching and 
resets.

I believe this is not applicable as on s390 we don't support any other 
PCI hotplug drivers given the unique nature of zPCI architecture.

Pre-existing issues:
- [High] Lockless access to `dev->slot` in `pci_dev_reset_slot_function` 
can lead to Use-After-Free if a hotplug driver is concurrently unbound.

Sashiko identified this as a pre-existing issue, so I don't think should 
be addressed with this patch.


Thanks

Farhan

Re: [PATCH v21 1/4] PCI: Allow per function PCI slots to fix slot reset on s390

[Farhan Ali]

On 7/15/2026 4:34 PM, Bjorn Helgaas wrote:
>> On s390 systems, which use a machine level hypervisor, PCI devices are
>> always accessed through a form of PCI pass-through which fundamentally
>> operates on a per PCI function granularity. This is also reflected in the
>> s390 PCI hotplug driver which creates hotplug slots for individual PCI
>> functions. Its reset_slot() function, which is a wrapper for
>> zpci_hot_reset_device(), thus also resets individual functions.
>>
>> Currently, the kernel's PCI_SLOT() macro assigns the same pci_slot object
>> to multifunction devices.
> PCI_SLOT() doesn't assign pci_slot objects; I guess they're assigned
> by some code that*uses* PCI_SLOT().  Since this says "currently," I
> assume you're changing that code, so we should mention where it is to
> help readers out.

Thanks for your response! I can re-word the commit message, how about 
something like this:

Currently, the pci_create_slot() assigns the same pci_slot object to 
multifunction devices.


>
> I see some Sashiko comments; those also need to be addressed or
> explained away.

Regarding Sashiko's comments for this patch, it mentions 2 issues:

New issues: - [High] Unconditional enablement of `per_func_slot` on S390 
breaks standard PCI hotplug (e.g., pciehp, shpchp) slot matching and 
resets.

I believe this is not applicable as on s390 we don't support any other 
PCI hotplug drivers given the unique nature of zPCI architecture.

Pre-existing issues:
- [High] Lockless access to `dev->slot` in `pci_dev_reset_slot_function` 
can lead to Use-After-Free if a hotplug driver is concurrently unbound.

Sashiko identified this as a pre-existing issue, so I don't think should 
be addressed with this patch.


Thanks

Farhan