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Subject: Re: [PATCH 5/5] KVM: VMX: Handle RFLAGS.TF in
 skip_emulated_instruction
To: Kyle Huey <me@kylehuey.com>
References: <20161128041856.11420-1-khuey@kylehuey.com>
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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Mon, 28 Nov 2016 18:19:23 +0100
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On 28/11/2016 17:13, Kyle Huey wrote:
> On Mon, Nov 28, 2016 at 3:42 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> This code is pretty much the same as kvm_vcpu_check_singlestep.  Let's
>> not duplicate the code and implement skip_emulated_instruction can be
>> implemented in x86.c, like
>>
>>         unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
>>         int r = EMULATE_DONE;
>>
>>         /* This would be the no_trap variant */
>>         kvm_x86_ops->skip_emulated_instruction(vcpu);
>>         kvm_vcpu_check_singlestep(vcpu, rflags, &r);
>>         return r == EMULATE_DONE;
>>
>> (because x86.c/vmx.c/svm.c are separate modules, when moving the function
>> to x86.c you should rename it to kvm_skip_emulated_instruction).
>>
>> Paolo
> 
> They're not exactly the same.  For some reason I don't understand
> kvm_vcpu_check_singlestep clears the trap flag.  Perhaps that is also
> a bug?

The Intel manual says "The processor clears the TF flag before calling
the exception handler" (17.3.1.4), so I think you should do it too.

Paolo