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

[Alex Williamson]

On Sun, 1 Dec 2024 22:11:29 -0700
Alex Williamson <alex.williamson@redhat.com> wrote:

> 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,

Disregard this latter comment, I just wasn't copied on the cover letter
and didn't have it in my inbox to root the thread.  Thanks,

Alex