linux-kernel - RE: [PATCH 1/5] x86/pat: Let pat_pfn_immune_to_uc_mtrr() check MTRR for untracked PAT range

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Date: Mon, 20 May 2024 02:36:20 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: "Zhao, Yan Y" <yan.y.zhao@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
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	<yi.l.liu@...el.com>, Tom Lendacky <thomas.lendacky@....com>
Subject: RE: [PATCH 1/5] x86/pat: Let pat_pfn_immune_to_uc_mtrr() check MTRR
 for untracked PAT range

> From: Sean Christopherson <seanjc@...gle.com>
> Sent: Thursday, May 16, 2024 10:07 PM
> 
> +Tom
> 
> On Thu, May 16, 2024, Kevin Tian wrote:
> > > From: Zhao, Yan Y <yan.y.zhao@...el.com>
> > > 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@...el.com>
> > > > > 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().

yes that sounds clearer. Checking MMIO vs. RAM is kind of indirect hint.

> 
> 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()?

yes, better.