Re: [PATCH v2 1/2] KVM: arm64: Add handler for MOPS exceptions

[Marc Zyngier]

On Mon, 02 Oct 2023 15:06:33 +0100,
Kristina Martsenko <kristina.martsenko@xxxxxxx> wrote:
> 
> On 29/09/2023 10:23, Marc Zyngier wrote:
> > On Wed, 27 Sep 2023 09:28:20 +0100,
> > Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
> >>
> >> On Mon, Sep 25, 2023 at 04:16:06PM +0100, Kristina Martsenko wrote:
> >>
> >> [...]
> >>
> >>>> What is the rationale for advancing the state machine? Shouldn't we
> >>>> instead return to the guest and immediately get the SS exception,
> >>>> which in turn gets reported to userspace? Is it because we rollback
> >>>> the PC to a previous instruction?
> >>>
> >>> Yes, because we rollback the PC to the prologue instruction. We advance the
> >>> state machine so that the SS exception is taken immediately upon returning to
> >>> the guest at the prologue instruction. If we didn't advance it then we would
> >>> return to the guest, execute the prologue instruction, and then take the SS
> >>> exception on the middle instruction. Which would be surprising as userspace
> >>> would see the middle and epilogue instructions executed multiple times but not
> >>> the prologue.
> >>
> >> I agree with Kristina that taking the SS exception on the prologue is
> >> likely the best course of action. Especially since it matches the
> >> behavior of single-stepping an EL0 MOPS sequence with an intervening CPU
> >> migration.
> >>
> >> This behavior might throw an EL1 that single-steps itself for a loop,
> >> but I think it is impossible for a hypervisor to hide the consequences
> >> of vCPU migration with MOPS in the first place.
> >>
> >> Marc, I'm guessing you were most concerned about the former case where
> >> the VMM was debugging the guest. Is there something you're concerned
> >> about I missed?
> > 
> > My concern is not only the VMM, but any userspace that perform
> > single-stepping. Imagine the debugger tracks PC by itself, and simply
> > increments it by 4 on a non-branch, non-fault instruction.
> > 
> > Move the vcpu or the userspace around, rewind PC, and now the debugger
> > is out of whack with what is executing. While I agree that there is
> > not much a hypervisor can do about that, I'm a bit worried that we are
> > going to break existing SW with this.
> > 
> > Now the obvious solution is "don't do that"...
> 
> If the debugger can handle the PC changing on branching or faulting
> instructions, then why can't it handle it on MOPS instructions? Wouldn't
> such a debugger need to be updated any time the architecture adds new
> branching or faulting instructions? What's different here?

What is different is that we *go back* in the instruction stream,
which is a first. I'm not saying that the debugger I describe above
would be a very clever piece of SW, quite the opposite. But the way
the architecture works results in some interesting side-effects, and
I'm willing to bet that some SW will break (rr?).

But again, asymmetric systems are such a bad idea that I can't say I
care.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.