linux-kernel - Re: [PATCH 16/22] KVM: x86: Remove manual pfn lookup when retrying #PF after failed emulation

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Message-ID: <6530eb94-b937-415c-8457-f5c598d94e7b@redhat.com>
Date: Wed, 14 Aug 2024 19:50:01 +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 16/22] KVM: x86: Remove manual pfn lookup when retrying
 #PF after failed emulation

On 8/9/24 21:03, Sean Christopherson wrote:
> Drop the manual pfn look when retrying an instruction that KVM failed to
> emulation in response to a #PF due to a write-protected gfn.  Now that KVM
> sets EMULTYPE_PF if and only if the page fault it a write-protected gfn,

Pointing out where this happened will likely help a few years from now:

With the introduction of RET_PF_WRITE_PROTECTED, KVM sets EMULTYPE_PF if 
and only if the page fault it a write-protected gfn, i.e. if and only if 
there's a writable memslot.  KVM will never try to redo an instruction 
that failed on emulated MMIO (no slot, or a write to a read-only slot), 
so therefore there's no redo the lookup in reexecute_instruction().

Paolo

> i.e. if and only if there's a writable memslot, there's no need to redo
> the lookup to avoid retrying an instruction that failed on emulated MMIO
> (no slot, or a write to a read-only slot).
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/x86.c | 18 ------------------
>   1 file changed, 18 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 771e67381fce..67f9871990fb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8867,7 +8867,6 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   				  int emulation_type)
>   {
>   	gpa_t gpa = cr2_or_gpa;
> -	kvm_pfn_t pfn;
>   
>   	if (!(emulation_type & EMULTYPE_ALLOW_RETRY_PF))
>   		return false;
> @@ -8887,23 +8886,6 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   			return true;
>   	}
>   
> -	/*
> -	 * Do not retry the unhandleable instruction if it faults on the
> -	 * readonly host memory, otherwise it will goto a infinite loop:
> -	 * retry instruction -> write #PF -> emulation fail -> retry
> -	 * instruction -> ...
> -	 */
> -	pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
> -
> -	/*
> -	 * If the instruction failed on the error pfn, it can not be fixed,
> -	 * report the error to userspace.
> -	 */
> -	if (is_error_noslot_pfn(pfn))
> -		return false;
> -
> -	kvm_release_pfn_clean(pfn);
> -
>   	/*
>   	 * If emulation may have been triggered by a write to a shadowed page
>   	 * table, unprotect the gfn (zap any relevant SPTEs) and re-enter the