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

[Marc Zyngier]

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"...

	M.

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