Re: [PATCH 5/6] KVM: x86: Preserve injected TF across emulation

Jan Kiszka
2010-02-23T10:41:14Z

Gleb Natapov wrote:
> On Tue, Feb 23, 2010 at 11:13:13AM +0100, Jan Kiszka wrote:
>> Gleb Natapov wrote:
>>> On Mon, Feb 22, 2010 at 06:51:22PM +0100, Jan Kiszka wrote:
>>>> Call directly into the vendor services for getting/setting rflags in
>>>> emulate_instruction to ensure injected TF survives the emulation.
>>>>
>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>>> ---
>>>> arch/x86/kvm/x86.c | 4 ++--
>>>> 1 files changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>> index e2e03a4..19e8b28 100644
>>>> --- a/arch/x86/kvm/x86.c
>>>> +++ b/arch/x86/kvm/x86.c
>>>> @@ -3468,7 +3468,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
>>>> kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>>>>
>>>> vcpu->arch.emulate_ctxt.vcpu = vcpu;
>>>> - vcpu->arch.emulate_ctxt.eflags = kvm_get_rflags(vcpu);
>>>> + vcpu->arch.emulate_ctxt.eflags = kvm_x86_ops->get_rflags(vcpu);
>>> So now emulator runs with injected TF? Hmm, then may be emulator should
>>> inject DB when appropriate and caller of emulate_instruction() should
>>> emulate DB intercept if external debugging is going on?
>> That is what patch 6 aims at, both for external as well as
>> guest-internal debugging.
>>
> It does at the wrong level. It tries to do it above emulator, but only emulator
> knows what is the sate of instruction emulation and when emulation is completed.
> If at this point TF is set it inject DB trap. The code above checks if
> DB intercept is enabled when DB is injected and calls usual DB intercept
> path.

Sorry, can't follow, your description does not match the above code for me.

This patch just ensures that we do not loose TF across emulation and, by
itself, already improves guest debugging: it allows to step over
emulated blocks with one instruction offset. And it has no side effects
as the emulator does not evaluate TF yet (before patch 6).

Jan

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