linux-kernel - Re: [PATCH v2 1/6] KVM: x86/mmu: add a new mmu zap helper to indicate memtype changes

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Date:   Tue, 30 May 2023 09:32:21 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Chao Gao <chao.gao@...el.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <pbonzini@...hat.com>
Subject: Re: [PATCH v2 1/6] KVM: x86/mmu: add a new mmu zap helper to
 indicate memtype changes

On Thu, May 25, 2023 at 08:54:15AM -0700, Sean Christopherson wrote:
> On Thu, May 25, 2023, Yan Zhao wrote:
> > On Wed, May 24, 2023 at 07:50:24AM -0700, Sean Christopherson wrote:
> > > bool __kvm_mmu_honors_guest_mtrrs(struct kvm *kvm, bool vm_has_noncoherent_dma);
> > > 
> > > static inline bool kvm_mmu_honors_guest_mtrrs(struct kvm *kvm)
> > > {
> > > 	
> > > 	return __kvm_mmu_honors_guest_mtrrs(kvm, kvm_arch_has_noncoherent_dma(kvm));
> > > }
> > 
> > This should work and it centralizes the comments into one place, though I dislike
> > having to pass true as vm_has_noncoherent_dma in case of 1->0 transition. :)
> 
> Yeah, I don't love it either, but the whole 1=>0 transition is awkward.  FWIW,
> KVM doesn't strictly need to zap in that case since the guest isn't relying on
> WB for functionality, i.e. we could just skip it.
I think zap when 1=>0 transition is still useful.
E.g. if this non-coherent DMA is unassigned
(1) when CR0.CD=1 (KVM_X86_QUIRK_CD_NW_CLEARED is not enabled), or
(2) when CR0.CD=0 and MTRRs are disabled,
it's better to zap the UC ranges for better performance.