Re: [PATCH v3 07/11] KVM: VMX: drop IPAT in memtype when CD=1 for KVM_X86_QUIRK_CD_NW_CLEARED

[Xiaoyao Li]

On 6/20/2023 10:34 AM, Yan Zhao wrote:
> On Tue, Jun 20, 2023 at 10:42:57AM +0800, Chao Gao wrote:
> > On Fri, Jun 16, 2023 at 10:38:15AM +0800, Yan Zhao wrote:
> > > For KVM_X86_QUIRK_CD_NW_CLEARED, remove the ignore PAT bit in EPT memory
> > > types when cache is disabled and non-coherent DMA are present.
> > >
> > > With the quirk KVM_X86_QUIRK_CD_NW_CLEARED, WB + IPAT are returned as the
> > > EPT memory type when guest cache is disabled before this patch.
> > > Removing the IPAT bit in this patch will allow effective memory type to
> > > honor PAT values as well, which will make the effective memory type
> > Given guest sets CR0.CD, what's the point of honoring (guest) PAT? e.g.,
> > which guests can benefit from this change?
> This patch is actually a preparation for later patch 10 to implement
> fine-grained zap.
> If when CR0.CD=1 the EPT type is WB + IPAT, and
> when CR0.CD=0 + mtrr enabled, EPT type is WB or UC or ..., which are
> without IPAT, then we have to always zap all EPT entries.
>
> Given removing the IPAT bit when CR0.CD=1 only makes the quirk
> KVM_X86_QUIRK_CD_NW_CLEARED more strict (meaning it could be WC/UC... if
> the guest PAT overwrites it), it's still acceptable.

Per my understanding, the reason why KVM had KVM_X86_QUIRK_CD_NW_CLEARED 
is to ensure the memory type is WB to achieve better boot performance 
for old OVMF.

you need to justify the original purpose is not broken by this patch.