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


From: Alex Williamson
Subject: Re: [PATCH 1/8] vfio/igd: fix GTT stolen memory size calculation for gen 7
Date: Sun, 1 Dec 2024 22:11:29 -0700

On Mon,  2 Dec 2024 00:09:31 +0800
Tomita Moeko <tomitamoeko@gmail.com> wrote:

> 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) {
>          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;
>      }
>  

I'd argue this should be rolled into patch 4.  It's not really fixing
anything because igd_gen() can't return anything between 6 and 8.  This
only allows for several device versions that we currently consider to
be gen 6 to align with i915 kernel driver generation by calling them
generation 7.  We'd previously lumped them into generation 6 because
there's no functional difference we care about here between 6 & 7.

In the next patch you replace this '1 << ggms_mb' with '*= 2', which
would be equivalent to 'ggms_mb << 1' and matches your description that
the increment is doubled.  Is there a separate bug fix that needs to be
pulled out here?

Also, please send a cover letter for any series longer than a single
patch and please configure your tools to thread the series.  Thanks,

Alex




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