Re: [PATCH v3 03/10] PCI: Allow per function PCI slots

Farhan Ali
2025-09-16T18:38:07Z

On 9/15/2025 11:52 PM, Cédric Le Goater wrote:
> Hello Ali,
>
> On 9/11/25 20:33, 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. 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. 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.
>>
>> Fixes: 44510d6fa0c0 ("s390/pci: Handling multifunctions")
>> Suggested-by: Niklas Schnelle <schnelle@linux.ibm.com>
>> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
>> ---
>>   drivers/pci/hotplug/s390_pci_hpc.c | 10 ++++++++--
>>   drivers/pci/pci.c                  |  4 +++-
>>   drivers/pci/slot.c                 | 14 +++++++++++---
>>   include/linux/pci.h                |  1 +
>>   4 files changed, 23 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/pci/hotplug/s390_pci_hpc.c
>> b/drivers/pci/hotplug/s390_pci_hpc.c
>> index d9996516f49e..8b547de464bf 100644
>> --- a/drivers/pci/hotplug/s390_pci_hpc.c
>> +++ b/drivers/pci/hotplug/s390_pci_hpc.c
>> @@ -126,14 +126,20 @@ static const struct hotplug_slot_ops
>> s390_hotplug_slot_ops = {
>>     int zpci_init_slot(struct zpci_dev *zdev)
>>   {
>> +    int ret;
>>       char name[SLOT_NAME_SIZE];
>>       struct zpci_bus *zbus = zdev->zbus;
>>         zdev->hotplug_slot.ops = &s390_hotplug_slot_ops;
>>         snprintf(name, SLOT_NAME_SIZE, "%08x", zdev->fid);
>> -    return pci_hp_register(&zdev->hotplug_slot, zbus->bus,
>> -                   zdev->devfn, name);
>> +    ret = pci_hp_register(&zdev->hotplug_slot, zbus->bus,
>> +                zdev->devfn, name);
>> +    if (ret)
>> +        return ret;
>> +
>> +    zdev->hotplug_slot.pci_slot->per_func_slot = 1;
>> +    return 0;
>>   }
>>     void zpci_exit_slot(struct zpci_dev *zdev)
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index 3994fa82df68..70296d3b1cfc 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -5061,7 +5061,9 @@ 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 ||
>> +    if (dev->multifunction && !dev->slot->per_func_slot)
>> +        return -ENOTTY;
>> +    if (dev->subordinate || !dev->slot ||
>>           dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET)
>>           return -ENOTTY;
>>   diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
>> index 50fb3eb595fe..51ee59e14393 100644
>> --- a/drivers/pci/slot.c
>> +++ b/drivers/pci/slot.c
>> @@ -63,6 +63,14 @@ 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 PCI_SLOT(dev->devfn) == slot->number;
>> +}
>> +
>>   static void pci_slot_release(struct kobject *kobj)
>>   {
>>       struct pci_dev *dev;
>> @@ -73,7 +81,7 @@ static void pci_slot_release(struct kobject *kobj)
>>         down_read(&pci_bus_sem);
>>       list_for_each_entry(dev, &slot->bus->devices, bus_list)
>> -        if (PCI_SLOT(dev->devfn) == slot->number)
>> +        if (pci_dev_matches_slot(dev, slot))
>>               dev->slot = NULL;
>>       up_read(&pci_bus_sem);
>>   @@ -166,7 +174,7 @@ void pci_dev_assign_slot(struct pci_dev *dev)
>>         mutex_lock(&pci_slot_mutex);
>>       list_for_each_entry(slot, &dev->bus->slots, list)
>> -        if (PCI_SLOT(dev->devfn) == slot->number)
>> +        if (pci_dev_matches_slot(dev, slot))
>>               dev->slot = slot;
>>       mutex_unlock(&pci_slot_mutex);
>>   }
>> @@ -285,7 +293,7 @@ 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 (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 59876de13860..9265f32d9786 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -78,6 +78,7 @@ struct pci_slot {
>>       struct list_head    list;        /* Node in list of slots */
>>       struct hotplug_slot    *hotplug;    /* Hotplug info (move here) */
>>       unsigned char        number;        /* PCI_SLOT(pci_dev->devfn) */
>> +    unsigned int        per_func_slot:1; /* Allow per function slot */
>>       struct kobject        kobj;
>>   };
>
> This change generates a kernel oops on x86_64. It can be reproduced in
> a VM.
>
>
> C.
>
> [    3.073990] BUG: kernel NULL pointer dereference, address:
> 0000000000000021
> [    3.074976] #PF: supervisor read access in kernel mode
> [    3.074976] #PF: error_code(0x0000) - not-present page
> [    3.074976] PGD 0 P4D 0
> [    3.074976] Oops: Oops: 0000 [#1] SMP NOPTI
> [    3.074976] CPU: 18 UID: 0 PID: 1 Comm: swapper/0 Not tainted
> 6.17.0-rc6-clg-dirty #8 PREEMPT(voluntary)
> [    3.074976] Hardware name: Supermicro Super Server/X13SAE-F, BIOS
> 4.2 12/17/2024
> [    3.074976] RIP: 0010:pci_reset_bus_function+0xdf/0x160
> [    3.074976] Code: 4e 08 00 00 40 0f 85 83 00 00 00 48 8b 78 18 e8
> 27 9d ff ff 83 f8 e7 74 17 48 83 c4 08 5b 5d 41 5c c3 cc cc cc cc 48
> 8b 43 30 <f6> 40 21 01 75 b6 48 8b 53 10 48 83 7a 10 00 74 5e 48 83 7b
> 18 00
> [    3.074976] RSP: 0000:ffffcd808007b9a8 EFLAGS: 00010202
> [    3.074976] RAX: 0000000000000000 RBX: ffff88c4019b8000 RCX:
> 0000000000000000
> [    3.074976] RDX: 0000000000000000 RSI: 0000000000000001 RDI:
> ffff88c4019b8000
> [    3.074976] RBP: 0000000000000001 R08: 0000000000000002 R09:
> ffffcd808007b99c
> [    3.074976] R10: ffffcd808007b950 R11: 0000000000000000 R12:
> 0000000000000001
> [    3.074976] R13: ffff88c4019b80c8 R14: ffff88c401a7e028 R15:
> ffff88c401a73400
> [    3.074976] FS:  0000000000000000(0000) GS:ffff88d38aad5000(0000)
> knlGS:0000000000000000
> [    3.074976] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    3.074976] CR2: 0000000000000021 CR3: 0000000f66222001 CR4:
> 0000000000770ef0
> [    3.074976] PKRU: 55555554
> [    3.074976] Call Trace:
> [    3.074976]  <TASK>
> [    3.074976]  ? pci_pm_reset+0x39/0x180
> [    3.074976]  pci_init_reset_methods+0x52/0x80
> [    3.074976]  pci_device_add+0x215/0x5d0
> [    3.074976]  pci_scan_single_device+0xa2/0xe0
> [    3.074976]  pci_scan_slot+0x66/0x1c0
> [    3.074976]  ? klist_next+0x145/0x150
> [    3.074976]  pci_scan_child_bus_extend+0x3a/0x290
> [    3.074976]  acpi_pci_root_create+0x236/0x2a0
> [    3.074976]  pci_acpi_scan_root+0x19b/0x1f0
> [    3.074976]  acpi_pci_root_add+0x1a5/0x370
> [    3.074976]  acpi_bus_attach+0x1a8/0x290
> [    3.074976]  ? __pfx_acpi_dev_for_one_check+0x10/0x10
> [    3.074976]  device_for_each_child+0x4b/0x80
> [    3.074976]  acpi_dev_for_each_child+0x28/0x40
> [    3.074976]  ? __pfx_acpi_bus_attach+0x10/0x10
> [    3.074976]  acpi_bus_attach+0x7a/0x290
> [    3.074976]  ? _raw_spin_unlock_irqrestore+0x23/0x40
> [    3.074976]  ? __pfx_acpi_dev_for_one_check+0x10/0x10
> [    3.074976]  device_for_each_child+0x4b/0x80
> [    3.074976]  acpi_dev_for_each_child+0x28/0x40
> [    3.074976]  ? __pfx_acpi_bus_attach+0x10/0x10
> [    3.074976]  acpi_bus_attach+0x7a/0x290
> [    3.074976]  acpi_bus_scan+0x6a/0x1c0
> [    3.074976]  ? __pfx_acpi_init+0x10/0x10
> [    3.074976]  acpi_scan_init+0xdc/0x280
> [    3.074976]  ? __pfx_acpi_init+0x10/0x10
> [    3.074976]  acpi_init+0x218/0x530
> [    3.074976]  do_one_initcall+0x40/0x310
> [    3.074976]  kernel_init_freeable+0x2fe/0x450
> [    3.074976]  ? __pfx_kernel_init+0x10/0x10
> [    3.074976]  kernel_init+0x16/0x1d0
> [    3.074976]  ret_from_fork+0x1ab/0x1e0
> [    3.074976]  ? __pfx_kernel_init+0x10/0x10
> [    3.074976]  ret_from_fork_asm+0x1a/0x30
> [    3.074976]  </TASK>
> [    3.074976] Modules linked in:
> [    3.074976] CR2: 0000000000000021
> [    3.074976] ---[ end trace 0000000000000000 ]---
> [    3.074976] RIP: 0010:pci_reset_bus_function+0xdf/0x160
>
Hi Cedric,

Thanks for pointing this out. I missed that dev->slot could be NULL and
so the per_func_slot check should be done after the check for
!dev->slot. I tried this change on top of the patch in an x86_64 VM and
was able to boot the VM without the oops.

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 70296d3b1cfc..3631f7faa0cf 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -5061,10 +5061,9 @@ 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->slot->per_func_slot)
-               return -ENOTTY;
if (dev->subordinate || !dev->slot ||
-           dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET)
+           dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET ||
+           (dev->multifunction && !dev->slot->per_func_slot))
return -ENOTTY;

Thanks
Farhan