Re: [PATCH v2] vfio/pci: Introduce x-pci-class-code option

[Tomita Moeko <tomitamoeko@gmail.com>]

On 2025/5/29 2:30, Alex Williamson wrote:
> On Wed, 28 May 2025 23:55:48 +0800
> Tomita Moeko <tomitamoeko@gmail.com> wrote:
> 
>> Introduce x-pci-class-code option to allow users to override PCI class
>> code of a device, similar to the existing x-pci-vendor-id option. Only
>> the lower 24 bits of this option are used, though a uint32 is used here
>> for determining whether the value is valid and set by user.
>>
>> Additionally, to prevent exposing VGA ranges on non-VGA devices, the
>> x-vga=on option requires x-pci-class-code is either unset or set to
>> VGA controller class.
>>
>> This is mainly intended for IGD devices that expose themselves either
>> as VGA controller (primary display) or Display controller (non-primary
>> display). The UEFI GOP driver depends on the device reporting a VGA
>> controller class code (0x030000).
>>
>> Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
>> ---
>> v2:
>> * Add vdev class code check in vfio_populate_vga().
>> * Fix type in trace-events.
>> Link: https://lore.kernel.org/all/20250524153102.19747-1-tomitamoeko@gmail.com/
>>
>>  hw/vfio/pci.c        | 25 +++++++++++++++++++++++++
>>  hw/vfio/pci.h        |  1 +
>>  hw/vfio/trace-events |  1 +
>>  3 files changed, 27 insertions(+)
>>
>> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
>> index b1250d85bf..d57cb7356e 100644
>> --- a/hw/vfio/pci.c
>> +++ b/hw/vfio/pci.c
>> @@ -2726,6 +2726,14 @@ bool vfio_populate_vga(VFIOPCIDevice *vdev, Error **errp)
>>          return false;
>>      }
>>  
>> +    /* vdev class should be either unmodified (PCI_ANY_ID), or VGA controller */
>> +    if ((vdev->class_code != PCI_ANY_ID) &&
>> +        (vdev->class_code != (PCI_CLASS_DISPLAY_VGA << 8)) &&
>> +        (vdev->class_code != (PCI_CLASS_NOT_DEFINED_VGA << 8))) {
>> +        error_setg(errp, "vdev is not a VGA device");
>> +        return false;
>> +    }
> 
> I think we should follow the scheme used for vendor_id and device_id to
> populate the struct field when not specified.  That let's us use it
> more easily and consistently for things like this.

Hi, Alex

The class code override takes place in vfio_pci_config_setup(), where
is after vfio_populate_vga() is called in vfio_populate_device(). So
I have to check if it equals to default or VGA class code here, and
not initializing the sturct field with device value. If we decide to
initialize it for other purpose, I personally think we should also
save the subvendor/subdevice value as well.

> I think we can ignore PCI_CLASS_NOT_DEFINED_VGA.  The PCI Local Bus
> Specification 2.1, dated June 1, 1995 (earliest I can find on the SIG)
> includes the VGA class code and specifies base class 0x00 is reserved
> for compatibility with devices built before the base class field was
> defined, so at least before 1995.  Also, neither the kernel or QEMU
> is_vga helpers account for this, so they'd not have a VGA region or be
> properly detected elsewhere.  Thanks,
> 
> Alex

Got it, will remove this in v3.

Thanks,
Moeko
 
>> +
>>      if (!(reg_info->flags & VFIO_REGION_INFO_FLAG_READ) ||
>>          !(reg_info->flags & VFIO_REGION_INFO_FLAG_WRITE) ||
>>          reg_info->size < 0xbffff + 1) {
>> @@ -3092,6 +3100,21 @@ static bool vfio_pci_config_setup(VFIOPCIDevice *vdev, Error **errp)
>>                                                vdev->sub_device_id);
>>      }
>>  
>> +    /*
>> +     * Class code is a 24-bit value at config space 0x09. Allow overriding it
>> +     * with any 24-bit value.
>> +     */
>> +    if (vdev->class_code != PCI_ANY_ID) {
>> +        if (vdev->class_code > 0xffffff) {
>> +            error_setg(errp, "invalid PCI class code provided");
>> +            return false;
>> +        }
>> +        /* Higher 24 bits of PCI_CLASS_REVISION are class code */
>> +        vfio_add_emulated_long(vdev, PCI_CLASS_REVISION,
>> +                               vdev->class_code << 8, ~0xff);
>> +        trace_vfio_pci_emulated_class_code(vbasedev->name, vdev->class_code);
>> +    }
>> +
>>      /* QEMU can change multi-function devices to single function, or reverse */
>>      vdev->emulated_config_bits[PCI_HEADER_TYPE] =
>>                                                PCI_HEADER_TYPE_MULTI_FUNCTION;
>> @@ -3489,6 +3512,8 @@ static const Property vfio_pci_dev_properties[] = {
>>                         sub_vendor_id, PCI_ANY_ID),
>>      DEFINE_PROP_UINT32("x-pci-sub-device-id", VFIOPCIDevice,
>>                         sub_device_id, PCI_ANY_ID),
>> +    DEFINE_PROP_UINT32("x-pci-class-code", VFIOPCIDevice,
>> +                       class_code, PCI_ANY_ID),
>>      DEFINE_PROP_UINT32("x-igd-gms", VFIOPCIDevice, igd_gms, 0),
>>      DEFINE_PROP_UNSIGNED_NODEFAULT("x-nv-gpudirect-clique", VFIOPCIDevice,
>>                                     nv_gpudirect_clique,
>> diff --git a/hw/vfio/pci.h b/hw/vfio/pci.h
>> index 5ce0fb916f..587eb8cc9a 100644
>> --- a/hw/vfio/pci.h
>> +++ b/hw/vfio/pci.h
>> @@ -156,6 +156,7 @@ struct VFIOPCIDevice {
>>      uint32_t device_id;
>>      uint32_t sub_vendor_id;
>>      uint32_t sub_device_id;
>> +    uint32_t class_code;
>>      uint32_t features;
>>  #define VFIO_FEATURE_ENABLE_VGA_BIT 0
>>  #define VFIO_FEATURE_ENABLE_VGA (1 << VFIO_FEATURE_ENABLE_VGA_BIT)
>> diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
>> index e90ec9bff8..e8d585b49a 100644
>> --- a/hw/vfio/trace-events
>> +++ b/hw/vfio/trace-events
>> @@ -46,6 +46,7 @@ vfio_pci_emulated_vendor_id(const char *name, uint16_t val) "%s 0x%04x"
>>  vfio_pci_emulated_device_id(const char *name, uint16_t val) "%s 0x%04x"
>>  vfio_pci_emulated_sub_vendor_id(const char *name, uint16_t val) "%s 0x%04x"
>>  vfio_pci_emulated_sub_device_id(const char *name, uint16_t val) "%s 0x%04x"
>> +vfio_pci_emulated_class_code(const char *name, uint32_t val) "%s 0x%06x"
>>  
>>  # pci-quirks.c
>>  vfio_quirk_rom_in_denylist(const char *name, uint16_t vid, uint16_t did) "%s %04x:%04x"
>