linux-kernel - Re: [PATCH 06/22] KVM: x86: Get RIP from vCPU state when storing it to last_retry

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Message-ID: <d4751f47-c5ca-4aa6-b114-086df25c4ce5@redhat.com>
Date: Wed, 14 Aug 2024 19:01:20 +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 06/22] KVM: x86: Get RIP from vCPU state when storing it
 to last_retry_eip

On 8/9/24 21:03, Sean Christopherson wrote:
> Read RIP from vCPU state instead of pulling it from the emulation context
> when filling last_retry_eip, which is part of the anti-infinite-loop
> protection used when unprotecting and retrying instructions that hit a
> write-protected gfn.
> 
> This will allow reusing the anti-infinite-loop protection in flows that
> never make it into the emulator.
> 
> This is a glorified nop as ctxt->eip is set to kvm_rip_read() in
> init_emulate_ctxt(), and EMULTYPE_PF emulation is mutually exclusive with
> EMULTYPE_NO_DECODE and EMULTYPE_SKIP, i.e. always goes through
> x86_decode_emulated_instruction() and hasn't advanced ctxt->eip (yet).

This is as much a nit as it can be, but "glorified nop" would be 
interpreted more as "the assignment is not needed at all", or something 
similarly wrong.  Just "This has no functional change because..." will do.

Paolo

> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/x86.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2072cceac68f..372ed3842732 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8973,7 +8973,7 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
>   	if (!kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa)))
>   		return false;
>   
> -	vcpu->arch.last_retry_eip = ctxt->eip;
> +	vcpu->arch.last_retry_eip = kvm_rip_read(vcpu);
>   	vcpu->arch.last_retry_addr = cr2_or_gpa;
>   	return true;
>   }