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:   Sun, 25 Jun 2023 15:14:37 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Yan Zhao <yan.y.zhao@...el.com>, Chao Gao <chao.gao@...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 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.

you need to justify the original purpose is not broken by this patch.