Re: [PATCH 1/5] x86/pat: Let pat_pfn_immune_to_uc_mtrr() check MTRR for untracked PAT range

[Sean Christopherson]

+Tom

On Thu, May 16, 2024, Kevin Tian wrote:
> > From: Zhao, Yan Y <yan.y.zhao@xxxxxxxxx>
> > Sent: Tuesday, May 7, 2024 5:13 PM
> > 
> > On Tue, May 07, 2024 at 04:26:37PM +0800, Tian, Kevin wrote:
> > > > From: Zhao, Yan Y <yan.y.zhao@xxxxxxxxx>
> > > > Sent: Tuesday, May 7, 2024 2:19 PM
> > > >
> > > > @@ -705,7 +705,17 @@ static enum page_cache_mode
> > > > lookup_memtype(u64 paddr)
> > > >   */
> > > >  bool pat_pfn_immune_to_uc_mtrr(unsigned long pfn)
> > > >  {
> > > > -	enum page_cache_mode cm = lookup_memtype(PFN_PHYS(pfn));
> > > > +	u64 paddr = PFN_PHYS(pfn);
> > > > +	enum page_cache_mode cm;
> > > > +
> > > > +	/*
> > > > +	 * Check MTRR type for untracked pat range since lookup_memtype()
> > > > always
> > > > +	 * returns WB for this range.
> > > > +	 */
> > > > +	if (x86_platform.is_untracked_pat_range(paddr, paddr + PAGE_SIZE))
> > > > +		cm = pat_x_mtrr_type(paddr, paddr + PAGE_SIZE,
> > > > _PAGE_CACHE_MODE_WB);
> > >
> > > doing so violates the name of this function. The PAT of the untracked
> > > range is still WB and not immune to UC MTRR.
> > Right.
> > Do you think we can rename this function to something like
> > pfn_of_uncachable_effective_memory_type() and make it work under
> > !pat_enabled() too?
> 
> let's hear from x86/kvm maintainers for their opinions.
> 
> My gut-feeling is that kvm_is_mmio_pfn() might be moved into the
> x86 core as the logic there has nothing specific to kvm itself. Also
> naming-wise it doesn't really matter whether the pfn is mmio. The
> real point is to find the uncacheble memtype in the primary mmu
> and then follow it in KVM.

Yeaaaah, we've got an existing problem there.  When AMD's SME is enabled, KVM
uses kvm_is_mmio_pfn() to determine whether or not to map memory into the guest
as encrypted or plain text.  I.e. KVM really does try to use this helper to
detect MMIO vs. RAM.  I highly doubt that actually works in all setups.

For SME, it seems like the best approach would be grab the C-Bit from the host
page tables, similar to how KVM uses host_pfn_mapping_level().

SME aside, I don't have objection to moving kvm_is_mmio_pfn() out of KVM.

> from that point probably a pfn_memtype_uncacheable() reads clearer.

or even just pfn_is_memtype_uc()?