Re: [PATCH 1/9] vfio/igd: Restrict legacy mode to Gen6-9 devices

[Tomita Moeko <tomitamoeko@gmail.com>]

On 5/5/25 16:42, Corvin Köhne wrote:
> On Tue, 2025-04-29 at 00:09 +0800, Tomita Moeko wrote:
>> CAUTION: External Email!!
>> Intel only provides legacy VBIOS for IGD up to Gen9, and there is no
>> CSM support on later devices. Additionally, Seabios can only handle
>> 32-bit BDSM register used until Gen9. Since legacy mode requires VGA
>> capability, restrict it to Gen6 through Gen9 devices.
>>
>> Link:
>> https://nospamproxywebp.beckhoff.com/enQsig/link?id=BAgAAADAsHNi0zEFiakAAACORk589NFmMEL0IRdt7DRVpnoi_6WKuXqQANgyXom23JAMNMr3UlEVSZvZdOtespZyqyGqsgLV2z0x8MkoDwNY9VFWuoC1hAOa_kpJihDYFTkqJw9sxp1uw6X9Oml3OGmOnrY8bVlHo2n7h5uiu73mh7RdgB0cFnBy59ExBLmCowsh6nXK5LA_Udnf-WbH0pzMWtCwjhPF453hx1Rm93_KYzUhuyswu_sg0
>>  
>> Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
>> ---
>>  docs/igd-assign.txt | 1 +
>>  hw/vfio/igd.c       | 6 +++++-
>>  2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/docs/igd-assign.txt b/docs/igd-assign.txt
>> index 3aed7956d5..95beb76812 100644
>> --- a/docs/igd-assign.txt
>> +++ b/docs/igd-assign.txt
>> @@ -47,6 +47,7 @@ Intel document [1] shows how to dump VBIOS to file. For UEFI
>> Option ROM, see
>>  
>>  QEMU also provides a "Legacy" mode that implicitly enables full functionality
>>  on IGD, it is automatically enabled when
>> +* IGD generation is 6 to 9 (Sandy Bridge to Comet Lake)
>>  * Machine type is i440fx
>>  * IGD is assigned to guest BDF 00:02.0
>>  * ROM BAR or romfile is present
>> diff --git a/hw/vfio/igd.c b/hw/vfio/igd.c
>> index 6678e0e5cd..cecc3245b7 100644
>> --- a/hw/vfio/igd.c
>> +++ b/hw/vfio/igd.c
>> @@ -516,11 +516,13 @@ static bool vfio_pci_igd_config_quirk(VFIOPCIDevice
>> *vdev, Error **errp)
>>  
>>      /*
>>       * For backward compatibility, enable legacy mode when
>> +     * - Device geneation is 6 to 9 (including both)
>>       * - Machine type is i440fx (pc_piix)
>>       * - IGD device is at guest BDF 00:02.0
>>       * - Not manually disabled by x-igd-legacy-mode=off
>>       */
>>      if ((vdev->igd_legacy_mode != ON_OFF_AUTO_OFF) &&
>> +        (gen >= 6 && gen <= 9) &&
>>          !strcmp(MACHINE_GET_CLASS(qdev_get_machine())->family, "pc_piix") &&
>>          (&vdev->pdev == pci_find_device(pci_device_root_bus(&vdev->pdev),
>>          0, PCI_DEVFN(0x2, 0)))) {
>> @@ -565,7 +567,9 @@ static bool vfio_pci_igd_config_quirk(VFIOPCIDevice *vdev,
>> Error **errp)
>>          vdev->features |= VFIO_FEATURE_ENABLE_IGD_LPC;
>>      } else if (vdev->igd_legacy_mode == ON_OFF_AUTO_ON) {
>>          error_setg(&err,
>> -                   "Machine is not i440fx or assigned BDF is not 00:02.0");
>> +                   "Machine is not i440fx, assigned BDF is not 00:02.0, "
>> +                   "or device %04x (gen %d) doesn't support legacy mode",
>> +                   vdev->device_id, gen);
>>          goto error;
>>      }
>>  
> 
> Afaik, at least the LPC quirk is required for Gen 10 and above for the GOP to
> work properly. On the other hand, Intel hasn't published any GOP yet, so it
> might be unlikely that user use it. So, not sure if this is a breaking change or
> not.
> 
> Besides that:
> 
> Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
> 

A previous change has added a new experimental option x-igd-lpc to allow
users manually enable the LPC bridge quirk, the documentation was also
updated with "GOP driver depends on LPC bridge present". In non-legacy
mode, user can manually enable lpc quirk with x-igd-lpc=on option.

Thanks,
Moeko