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

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Message-ID: <87blx1dy2z.fsf@vitty.brq.redhat.com>
Date:   Wed, 07 Aug 2019 13:17:08 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> 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.
>

(I'm always wondering when is the right time to add "plus a bunch of
miscellaneous fixes all over" to the PATCH0's Subject line :-)

Will do in the next version, thanks!

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

-- 
Vitaly