Re: [PATCH 1/8] vfio/igd: fix GTT stolen memory size calculation for gen 7

[Corvin Köhne]

On Mon, 2024-12-02 at 00:09 +0800, Tomita Moeko wrote:
> CAUTION: External Email!!
> Both intel documentation [1][2] and i915 driver shows GGMS represents
> GTT stolen memory size in multiple of 1MB, not 2MB starting from gen
> 8.
> 
> [1]
> https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/3rd-gen-core-desktop-vol-2-datasheet.pdf
> [2]
> https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/4th-gen-core-family-desktop-vol-2-datasheet.pdf
> 
> Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
> ---
>  hw/vfio/igd.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/vfio/igd.c b/hw/vfio/igd.c
> index 4047f4f071..e40e601026 100644
> --- a/hw/vfio/igd.c
> +++ b/hw/vfio/igd.c
> @@ -268,7 +268,7 @@ static int vfio_igd_gtt_max(VFIOPCIDevice *vdev)
>  
>      gmch = vfio_pci_read_config(&vdev->pdev, IGD_GMCH, sizeof(gmch));
>      ggms = (gmch >> (gen < 8 ? 8 : 6)) & 0x3;
> -    if (gen > 6) {
> +    if (gen > 7) {

This seems odd. The commit message talks about gen 8 but it changes the
behaviour for gen 7 only. Additionally, ggms_mb is still shifted for gen
8 and later, so it's still increment by steps of 2 MB. Shouldn't this be
something like gen < 8?

>          ggms = 1 << ggms;
>      }
>  
> @@ -678,7 +678,7 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice
> *vdev, int nr)
>  
>      /* Determine the size of stolen memory needed for GTT */
>      ggms_mb = (gmch >> (gen < 8 ? 8 : 6)) & 0x3;
> -    if (gen > 6) {
> +    if (gen > 7) {
>          ggms_mb = 1 << ggms_mb;
>      }
>  

Btw. you could consider adding a link to the source code of i915 too.


-- 
Kind regards,
Corvin
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