Messages in this thread | | | Date | Mon, 26 Jun 2023 08:08:20 +0800 | From | Yan Zhao <> | Subject | Re: [PATCH v3 07/11] KVM: VMX: drop IPAT in memtype when CD=1 for KVM_X86_QUIRK_CD_NW_CLEARED |
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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.
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