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

[Yuan Yao]

On Mon, Jun 26, 2023 at 08:08:20AM +0800, Yan Zhao wrote:
> On Sun, Jun 25, 2023 at 03:14:37PM +0800, Xiaoyao Li wrote:
> > 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.
> It works well for OVMF c9e5618f84b0cb54a9ac2d7604f7b7e7859b45a7,
> which is Apr 14 2015.
>
>
> > you need to justify the original purpose is not broken by this patch.
>
> Hmm, to dig into the history, the reason for this quirk is explained below:
>
> commit fb279950ba02e3210a16b11ecfa8871f3ee0ca49
> Author: Xiao Guangrong <guangrong.xiao@xxxxxxxxx>
> Date:   Thu Jul 16 03:25:56 2015 +0800
>
>     KVM: vmx: obey KVM_QUIRK_CD_NW_CLEARED
>
>     OVMF depends on WB to boot fast, because it only clears caches after
>     it has set up MTRRs---which is too late.
>
>     Let's do writeback if CR0.CD is set to make it happy, similar to what
>     SVM is already doing.
>
>
> which means WB is only a must for fast boot before OVMF has set up MTRRs.
> At that period, PAT is default to WB.
>
> After OVMF setting up MTRR, according to the definition of no-fill cache
> mode, "Strict memory ordering is not enforced unless the MTRRs are
> disabled and/or all memory is referenced as uncached", it's valid to
> honor PAT in no-fill cache mode.

Does it also mean that, the honor PAT in such no-fill cache mode should
also happen for non-quirk case ? e.g. the effective memory type can be
WC if EPT is UC + guest PAT is WC for CD=1.

> Besides, if the guest explicitly claim UC via PAT, why should KVM return
> WB?
> In other words, if it's still slow caused by a UC value in guest PAT,
> it's desired to be fixed in guest instead of a workaround in KVM.

the quirk may not work after this patch if the guest PAT is
stronger than WB for CD=1, we don't if any guest "works correctly" based
on this quirk, I hope no. How about highlight this in commit message
explicitly ?

Also I agree that such issue should be fixed in guest not in KVM.

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