linux-kernel - Re: [PATCH 1/5] KVM: x86: Remove VMX support for virtualizing guest MTRR memtypes

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Date: Thu, 14 Mar 2024 07:47:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dongli Zhang <dongli.zhang@...cle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Lai Jiangshan <jiangshanlai@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Josh Triplett <josh@...htriplett.org>, kvm@...r.kernel.org, 
	rcu@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Kevin Tian <kevin.tian@...el.com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Yiwei Zhang <zzyiwei@...gle.com>
Subject: Re: [PATCH 1/5] KVM: x86: Remove VMX support for virtualizing guest
 MTRR memtypes

On Thu, Mar 14, 2024, Dongli Zhang wrote:
> On 3/12/24 10:08, Sean Christopherson wrote:
> > On Mon, Mar 11, 2024, Dongli Zhang wrote:
> >> Since it is also controlled by other cases, e.g., kvm_arch_has_noncoherent_dma()
> >> at vmx_get_mt_mask(), it can be 'may_honor_guest_pat' too?
> >>
> >> Therefore, why not directly use 'shadow_memtype_mask' (without the API), or some
> >> naming like "ept_enabled_for_hardware".
> > 
> > Again, after this series, KVM will *always* honor guest PAT for CPUs with self-snoop,
> > i.e. KVM will *never* ignore guest PAT.  But for CPUs without self-snoop (or with
> > errata), KVM conditionally honors/ignores guest PAT.
> > 
> >> Even with the code from PATCH 5/5, we still have high chance that VM has
> >> non-coherent DMA?
> > 
> > I don't follow.  On CPUs with self-snoop, whether or not the VM has non-coherent
> > DMA (from VFIO!) is irrelevant.  If the CPU has self-snoop, then KVM can safely
> > honor guest PAT at all times.
> 
> 
> Thank you very much for the explanation.
> 
> According to my understanding of the explanation (after this series):
> 
> 1. When static_cpu_has(X86_FEATURE_SELFSNOOP) == true, it is 100% to "honor
> guest PAT".

Yes.

> 2. When static_cpu_has(X86_FEATURE_SELFSNOOP) == false (and
> shadow_memtype_mask), although only 50% chance (depending on where there is
> non-coherent DMA), at least now it is NOT 100% (to honor guest PAT) any longer.

Yes, though I wouldn't assign a percent probability to the non-coherent DMA case.

> Due to the fact it is not 100% (to honor guest PAT) any longer, there starts the
> trend (from 100% to 50%) to "ignore guest PAT", that is:
> kvm_mmu_may_ignore_guest_pat().

Yep.