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

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Date:   Thu, 25 May 2023 08:00:58 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Robert Hoo <robert.hoo.linux@...il.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 Thu, May 25, 2023, Robert Hoo wrote:
> On 5/23/2023 2:21 PM, Yan Zhao wrote:
> IIUC, your saving comes from skips the intermediate state during boot, when
> APs goes through setting MTRR, which would cause SPTE zap before your this
> patch set.
> 
> MHO was, now that your ignores other vCPU's MTRR settings (unless it is
> different from BP's MTRR?), why not let each vCPU's MTRR set/update directly
> set to the per-VM MTRR states (if differs from current value). It's guest
> OS/BIOS's responsibility to keep the consistency anyway. And even if the
> malfunction caused by the inconsistency might differ from that of native,
> SDM doesn't clearly state how the malfunction should be, does it?
> that's to say, anyone knows, when inconsistency happens, does it cause that
> logical processor malfunction or in fact it impacts the global MTRR
> settings? If the latter, I think leaving only the per-VM MTRR state aligns
> with native.

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.

> 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.