Re: [PATCH for-4.19] x86/EPT: relax iPAT for "invalid" MFNs

["Oleksii K." <oleksii.kurochko@xxxxxxxxx>]

On Mon, 2024-06-10 at 17:00 +0200, Jan Beulich wrote:
> On 10.06.2024 16:58, Jan Beulich wrote:
> > mfn_valid() is RAM-focused; it will often return false for MMIO.
> > Yet
> > access to actual MMIO space should not generally be restricted to
> > UC
> > only; especially video frame buffer accesses are unduly affected by
> > such
> > a restriction. Permit PAT use for directly assigned MMIO as long as
> > the
> > domain is known to have been granted some level of cache control.
> > 
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

~ Oleksii

> > ---
> > Considering that we've just declared PVH Dom0 "supported", this may
> > well
> > qualify for 4.19. The issue was specifically very noticable there.
> 
> Actually - meant to Cc Oleksii for this, and then forgot.
> 
> Jan
> 
> > The conditional may be more complex than really necessary, but it's
> > in
> > line with what we do elsewhere. And imo better continue to be a
> > little
> > too restrictive, than moving to too lax.
> > 
> > --- a/xen/arch/x86/mm/p2m-ept.c
> > +++ b/xen/arch/x86/mm/p2m-ept.c
> > @@ -503,7 +503,8 @@ int epte_get_entry_emt(struct domain *d,
> >  
> >      if ( !mfn_valid(mfn) )
> >      {
> > -        *ipat = true;
> > +        *ipat = type != p2m_mmio_direct ||
> > +                (!is_iommu_enabled(d) &&
> > !cache_flush_permitted(d));
> >          return X86_MT_UC;
> >      }
> >  
>