linux-kernel - Re: [PATCH 5/5] KVM: VMX: Handle RFLAGS.TF in skip_emulated

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Message-ID: <b4f7b9ae-0a5a-b7fe-deb6-750933c1ca18@redhat.com>
Date:   Mon, 28 Nov 2016 18:19:23 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Kyle Huey <me@...ehuey.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] KVM: VMX: Handle RFLAGS.TF in
 skip_emulated_instruction



On 28/11/2016 17:13, Kyle Huey wrote:
> On Mon, Nov 28, 2016 at 3:42 AM, Paolo Bonzini <pbonzini@...hat.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