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

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Date:   Mon, 28 Nov 2016 23:43:02 +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 19:34, Kyle Huey wrote:
>> > 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.
> The processor does this automatically. "When accessing an exception or
> interrupt handler through either an interrupt gate or a trap gate, the
> processor clears the TF flag in the EFLAGS register after it saves the
> contents of the EFLAGS register on the stack." (Vol 3, 6.12.1.2)
> Empirically, this holds when injecting an exception on VM entry. If
> you take the x86/debug.c test from kvm-unit-tests and inspect RFLAGS
> in handle_db (not regs->rflags, but the actual RFLAGS register while
> running the exception handler) the TF is clear. And, if you modify my
> patch to clear TF before returning, the single stepping ceases after
> the CPUID instruction because the TF was in fact cleared for good.

Ok, then that would be a bug in kvm_vcpu_check_singlestep (because
kvm_vcpu_check_singlestep is mostly interesting for real mode emulation,
I checked kvm_inject_realmode_interrupt and it clears TF too, in
__emulate_int_real).

Paolo