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

[Yan Zhao <yan.y.zhao@intel.com>]

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.