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. > > > >
