linux-kernel - Re: [PATCH v2 5/6] KVM: x86: Keep a per-VM MTRR state

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Message-ID: <4dc14170-8e22-9da7-30ec-a5597acf1f8e@gmail.com>
Date:   Fri, 26 May 2023 09:49:41 +0800
From:   Robert Hoo <robert.hoo.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Yan Zhao <yan.y.zhao@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com
Subject: Re: [PATCH v2 5/6] KVM: x86: Keep a per-VM MTRR state

On 5/25/2023 11:00 PM, Sean Christopherson wrote:
[...]
> The MTRRs are not system wide or per-package though, they are per logical CPU.
> Yes, they "need" to be consistent with respect to one another, but only when the
> CPU is actually accessing memory.  This is a big reason why trying to track MTRRs
> as a per-VM asset in KVM is so difficult/messy.  Software doesn't rendezvous all
> CPUs and then do the write on just one CPU, each CPU does its own writes more or
> less independently.

Ah, got it, thanks!

(Some things of each logical processor seems just a shadow of an 
consolidate global one, e.g. CR0.CD. Some things are really separated 
setting of each logical processor, e.g. MTRR above. Really unfathomable 😅)
> 
>> BTW, with regard to KVM_X86_QUIRK_CD_NW_CLEARED, I see svm honors it while
>> vmx doesn't before it clear CR0.CD/NW.
>>
>> svm_set_cr0():
>>
>> 	if (kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED))
>> 		hcr0 &= ~(X86_CR0_CD | X86_CR0_NW);
>>
>>
>> vmx_set_cr0():
>>
>> 	hw_cr0 = (cr0 & ~KVM_VM_CR0_ALWAYS_OFF);
>>
>> Perhaps vmx side can be fixed passingly?
> 
> Sadly, no.  SVM and VMX manage guest memtype completely differently.  VMX doesn't
> allow CR0.CD=1 when VMX is enabled, and so KVM needs to emulate CR0.CD via the EPT
> memtype.

OK, get it, thanks. Wasn't aware of this through SDM.