linux-kernel - Re: [PATCH v3 07/11] KVM: VMX: drop IPAT in memtype when CD=1 for KVM_X86_QUIRK_CD_NW

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Date:   Tue, 20 Jun 2023 11:34:43 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <pbonzini@...hat.com>, <seanjc@...gle.com>, <kai.huang@...el.com>,
        <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v3 07/11] KVM: VMX: drop IPAT in memtype when CD=1 for
 KVM_X86_QUIRK_CD_NW_CLEARED

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.