linux-kernel - Re: [PATCH 09/22] KVM: x86/mmu: Try "unprotect for retry" iff there are indirect SPs

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Message-ID: <e50240f9-a476-4ace-86aa-f2fd33fbe320@redhat.com>
Date: Wed, 14 Aug 2024 19:30:26 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Peter Gonda <pgonda@...gle.com>, Michael Roth <michael.roth@....com>,
 Vishal Annapurve <vannapurve@...gle.com>,
 Ackerly Tng <ackerleytng@...gle.com>
Subject: Re: [PATCH 09/22] KVM: x86/mmu: Try "unprotect for retry" iff there
 are indirect SPs

On 8/9/24 21:03, Sean Christopherson wrote:
> Try to unprotect shadow pages if and only if indirect_shadow_pages is non-
> zero, i.e. iff there is at least one protected such shadow page.  Pre-
> checking indirect_shadow_pages avoids taking mmu_lock for write when the
> gfn is write-protected by a third party, i.e. not for KVM shadow paging,
> and in the *extremely* unlikely case that a different task has already
> unprotected the last shadow page.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 09a42dc1fe5a..358294889baa 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2736,6 +2736,9 @@ bool kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa)
>   	gpa_t gpa = cr2_or_gpa;
>   	bool r;
>   
> +	if (!vcpu->kvm->arch.indirect_shadow_pages)
> +		return false;

indirect_shadow_pages is accessed without a lock, so here please add a 
note that, while it may be stale, a false negative will only cause KVM 
to skip the "unprotect and retry" optimization.  (This is preexisting in 
reexecute_instruction() and goes away in patch 18, if I'm pre-reading 
that part of the series correctly).

Bonus points for opportunistically adding a READ_ONCE() here and in 
kvm_mmu_track_write().

Paolo

>   	if (!vcpu->arch.mmu->root_role.direct)
>   		gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL);
>