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Subject: Re: [PATCH 5/5] KVM: VMX: Handle RFLAGS.TF in
 skip_emulated_instruction
To: Kyle Huey <me@kylehuey.com>
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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Mon, 28 Nov 2016 23:43:02 +0100
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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