linux-kernel - Re: [PATCH v2 2/5] x86: KVM: svm: avoid flooding logs when skip_emulated

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Date:   Tue, 6 Aug 2019 08:40:33 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 2/5] x86: KVM: svm: avoid flooding logs when
 skip_emulated_instruction() fails

On Tue, Aug 06, 2019 at 08:01:47AM +0200, Vitaly Kuznetsov wrote:
> When we're unable to skip instruction with kvm_emulate_instruction() we
> will not advance RIP and most likely the guest will get stuck as
> consequitive attempts to execute the same instruction will likely result
> in the same behavior.
> 
> As we're not supposed to see these messages under normal conditions, switch
> to pr_err_once().
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Reviewed-by: Jim Mattson <jmattson@...gle.com>
> ---
>  arch/x86/kvm/svm.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 7e843b340490..80f576e05112 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -782,7 +782,8 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
>  	if (!svm->next_rip) {
>  		if (kvm_emulate_instruction(vcpu, EMULTYPE_SKIP) !=
>  				EMULATE_DONE)
> -			printk(KERN_DEBUG "%s: NOP\n", __func__);
> +			pr_err_once("KVM: %s: unable to skip instruction\n",
> +				    __func__);

IMO the proper fix would be to change skip_emulated_instruction() to
return an int so that emulation failure can be reported back up the stack.
It's a relatively minor change as there are a limited number of call sites
to skip_emulated_instruction() in SVM and VMX.

>  		return;
>  	}
>  	if (svm->next_rip - kvm_rip_read(vcpu) > MAX_INST_SIZE)
> -- 
> 2.20.1
>