Re: [PATCH 2/5] KVM: x86/mmu: Fine-grained check of whether a invalid & RAM PFN is MMIO

[Yan Zhao]

On Tue, May 07, 2024 at 04:39:27PM +0800, Tian, Kevin wrote:
> > From: Zhao, Yan Y <yan.y.zhao@xxxxxxxxx>
> > Sent: Tuesday, May 7, 2024 2:20 PM
> > @@ -101,9 +101,21 @@ static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
> >  			 */
> >  			(!pat_enabled() ||
> > pat_pfn_immune_to_uc_mtrr(pfn));
> > 
> > +	/*
> > +	 * If the PFN is invalid and not RAM in raw e820 table, keep treating it
> > +	 * as MMIO.
> > +	 *
> > +	 * If the PFN is invalid and is RAM in raw e820 table,
> > +	 * - if PAT is not enabled, always treat the PFN as MMIO to avoid
> > futher
> > +	 *   checking of MTRRs.
> > +	 * - if PAT is enabled, treat the PFN as MMIO if its PAT is UC/WC/UC-
> > in
> > +	 *   primary MMU.
> > +	 * to prevent guest cacheable access to MMIO PFNs.
> > +	 */
> >  	return !e820__mapped_raw_any(pfn_to_hpa(pfn),
> >  				     pfn_to_hpa(pfn + 1) - 1,
> > -				     E820_TYPE_RAM);
> > +				     E820_TYPE_RAM) ? true :
> > +				     (!pat_enabled() ||
> > pat_pfn_immune_to_uc_mtrr(pfn));
> 
> Is it for another theoretical problem in case the primary
> mmu uses a non-WB type on a invalid RAM-type pfn so
> you want to do additional scrutiny here?
Yes. Another untold reason is that patch 3 does not do CLFLUSH to this type of
memory since it's mapped as uncacheable in primary MMU. I feel that it's better
to ensure guest will not access it in cacheable memory type either.