linux-kernel - Re: [PATCH 12/12] KVM: x86: do not unload MMU roots on all role changes

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Message-ID: <29e1edc7-9f9f-0663-997f-3416269b6a89@redhat.com>
Date:   Tue, 15 Feb 2022 09:17:30 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, mlevitsk@...hat.com, dmatlack@...gle.com
Subject: Re: [PATCH 12/12] KVM: x86: do not unload MMU roots on all role
 changes

On 2/14/22 20:24, Sean Christopherson wrote:
> On Mon, Feb 14, 2022, Paolo Bonzini wrote:
>> On 2/11/22 19:48, Sean Christopherson wrote:
>>> On Wed, Feb 09, 2022, Paolo Bonzini wrote:
>>>> -	kvm_mmu_unload(vcpu);
>>>>    	kvm_init_mmu(vcpu);
>>>> +	kvm_mmu_new_pgd(vcpu, vcpu->arch.cr3);
>>>
>>> This is too risky IMO, there are far more flows than just MOV CR0/CR4 that are
>>> affected, e.g. SMM transitions, KVM_SET_SREG, etc...
>
> I'm not concerned about the TLB flush aspects so much as the addition of
> kvm_mmu_new_pgd() in new paths.

Okay, yeah those are more complex and the existing ones are broken too.

>>>> -	if ((cr0 ^ old_cr0) & KVM_MMU_CR0_ROLE_BITS)
>>>> +	if ((cr0 ^ old_cr0) & KVM_MMU_CR0_ROLE_BITS) {
>>>> +		/* Flush the TLB if CR0 is changed 1 -> 0.  */
>>>> +		if ((old_cr0 & X86_CR0_PG) && !(cr0 & X86_CR0_PG))
>>>> +			kvm_mmu_unload(vcpu);
>>>
>>> Calling kvm_mmu_unload() instead of requesting a flush isn't coherent with respect
>>> to the comment, or with SMEP handling.  And the SMEP handling isn't coherent with
>>> respect to the changelog.  Please elaborate :-)
>>
>> Yep, will do (the CR0.PG=0 case is similar to the CR0.PCIDE=0 case below).
> 
> Oh, you're freeing all roots to ensure a future MOV CR3 with NO_FLUSH and PCIDE=1
> can't reuse a stale root.  That's necessary if and only if the MMU is shadowing
> the guest, non-nested TDP MMUs just need to flush the guest's TLB.  The same is
> true for the PCIDE case, i.e. we could optimize that too, though the main motivation
> would be to clarify why all roots are unloaded.

Yes.  Clarifying all this should be done before the big change to 
kvm_mmu_reset_context().

>> Using kvm_mmu_unload() avoids loading a cached root just to throw it away
>> immediately after,
> 
> The shadow paging case will throw it away, but not the non-nested TDP MMU case?

Yes, the TDP case is okay since the role is the same.  kvm_init_mmu() is 
enough.

>> but I can change this to a new KVM_REQ_MMU_UPDATE_ROOT flag that does
>>
>> 	kvm_mmu_new_pgd(vcpu, vcpu->arch.cr3);
> 
> I don't think that's necessary, I was just confused by the discrepancy.

It may not be necessary but it is clearer IMO.  Let me post a new patch.

Paolo