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

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

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@intel.com>
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.
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.