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Date:   Tue, 20 Jun 2023 11:19:27 +0800
From:   Yan Zhao <yan.y.zhao@intel.com>
To:     Chao Gao <chao.gao@intel.com>
CC:     <kvm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <pbonzini@redhat.com>, <seanjc@google.com>, <kai.huang@intel.com>,
        <robert.hoo.linux@gmail.com>
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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Reply-To: Yan Zhao <yan.y.zhao@intel.com>
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On Tue, Jun 20, 2023 at 11:34:43AM +0800, Chao Gao wrote:
> On Tue, Jun 20, 2023 at 10:34:29AM +0800, 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.
> 
> OK. The goal is to reduce the cost of toggling CR0.CD. The key is if KVM sets
> the IPAT, then when CR0.CD is cleared by guest, KVM has to zap _all_ EPT entries
> at least to clear IPAT.
> 
> Can kvm honor guest MTRRs as well when CR0.CD=1 && with the quirk? then later
> clearing CR0.CD needn't zap _any_ EPT entry. But the optimization is exactly the
> one removed in patch 6. Maybe I miss something important.
I replied it in another mail.