linux-kernel - Re: [PATCH 22/22] KVM: x86/mmu: Detect if unprotect will do anything based on invalid

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Message-ID: <5f8c0ca4-ae99-4d1c-8525-51c6f1096eaa@redhat.com>
Date: Wed, 14 Aug 2024 19:57:57 +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 22/22] KVM: x86/mmu: Detect if unprotect will do anything
 based on invalid_list

On 8/9/24 21:03, Sean Christopherson wrote:
> Explicitly query the list of to-be-zapped shadow pages when checking to
> see if unprotecting a gfn for retry has succeeded, i.e. if KVM should
> retry the faulting instruction.
> 
> Add a comment to explain why the list needs to be checked before zapping,
> which is the primary motivation for this change.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 300a47801685..50695eb2ee22 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2731,12 +2731,15 @@ bool __kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   			goto out;
>   	}
>   
> -	r = false;
>   	write_lock(&kvm->mmu_lock);
> -	for_each_gfn_valid_sp_with_gptes(kvm, sp, gpa_to_gfn(gpa)) {
> -		r = true;
> +	for_each_gfn_valid_sp_with_gptes(kvm, sp, gpa_to_gfn(gpa))
>   		kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
> -	}
> +
> +	/*
> +	 * Snapshot the result before zapping, as zapping will remove all list
> +	 * entries, i.e. checking the list later would yield a false negative.
> +	 */

Hmm, the comment is kinda overkill?  Maybe just

	/* Return whether there were sptes to zap.  */
	r = !list_empty(&invalid_test);

I'm not sure about patch 21 - I like the simple kvm_mmu_unprotect_page() 
function.  Maybe rename it to kvm_mmu_zap_gfn() and make it static in 
the same patch?

Either way, this small cleanup applies even if the function is not inlined.

Thanks,

Paolo
> +	r = !list_empty(&invalid_list);
>   	kvm_mmu_commit_zap_page(kvm, &invalid_list);
>   	write_unlock(&kvm->mmu_lock);
>