Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping

Jan Kiszka
2021-03-16T17:07:54Z

On 16.03.21 17:50, Sean Christopherson wrote:
> On Tue, Mar 16, 2021, Maxim Levitsky wrote:
>> On Tue, 2021-03-16 at 16:31 +0100, Jan Kiszka wrote:
>>> Back then, when I was hacking on the gdb-stub and KVM support, the
>>> monitor trap flag was not yet broadly available, but the idea to once
>>> use it was already there. Now it can be considered broadly available,
>>> but it would still require some changes to get it in.
>>>
>>> Unfortunately, we don't have such thing with SVM, even recent versions,
>>> right? So, a proper way of avoiding diverting event injections while we
>>> are having the guest in an "incorrect" state should definitely be the goal.
>> Yes, I am not aware of anything like monitor trap on SVM.
>>
>>>
>>> Given that KVM knows whether TF originates solely from guest debugging
>>> or was (also) injected by the guest, we should be able to identify the
>>> cases where your approach is best to apply. And that without any extra
>>> control knob that everyone will only forget to set.
>> Well I think that the downside of this patch is that the user might actually
>> want to single step into an interrupt handler, and this patch makes it a bit
>> more complicated, and changes the default behavior.
>
> Yes. And, as is, this also blocks NMIs and SMIs. I suspect it also doesn't
> prevent weirdness if the guest is running in L2, since IRQs for L1 will cause
> exits from L2 during nested_ops->check_events().
>
>> I have no objections though to use this patch as is, or at least make this
>> the new default with a new flag to override this.
>
> That's less bad, but IMO still violates the principle of least surprise, e.g.
> someone that is single-stepping a guest and is expecting an IRQ to fire will be
> all kinds of confused if they see all the proper IRR, ISR, EFLAGS.IF, etc...
> settings, but no interrupt.

From my practical experience with debugging guests via single step,
seeing an interrupt in that case is everything but handy and generally
also not expected (though logical, I agree). IOW: When there is a knob
for it, it will remain off in 99% of the time.

But I see the point of having some control, in an ideal world also an
indication that there are pending events, permitting the user to decide
what to do. But I suspect the gdb frontend and protocol does not easily
permit that.

>
>> Sean Christopherson, what do you think?
>
> Rather than block all events in KVM, what about having QEMU "pause" the timer?
> E.g. save MSR_TSC_DEADLINE and APIC_TMICT (or inspect the guest to find out
> which flavor it's using), clear them to zero, then restore both when
> single-stepping is disabled. I think that will work?
>

No one can stop the clock, and timers are only one source of interrupts.
Plus they do not all come from QEMU, some also from KVM or in-kernel
sources directly. Would quickly become a mess.

Jan

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