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

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Oct 02, 2023 at 03:55:33PM +0100, Marc Zyngier wrote:
> On Mon, 02 Oct 2023 15:06:33 +0100,
> Kristina Martsenko <kristina.martsenko@arm.com> wrote:
> > On 29/09/2023 10:23, Marc Zyngier wrote:
> > > On Wed, 27 Sep 2023 09:28:20 +0100,
> > > Oliver Upton <oliver.upton@linux.dev> 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?).

The way the architecture works, either with or without Kristina's
single-step change, a debugger would get confused. At least for EL0, I
find the proposed (well, upstreamed) approach more predictable - it
always restarts from the prologue in case of migration between CPUs with
different MOPS implementation (which is not just theoretical AFAIK).
It's more like these three instructions are a bigger CISC one ;) (though
the CPU can step through its parts).

A more transparent approach would have been to fully emulate the
instructions in the kernel and advance the PC as expected but I don't
think that's even possible. An implementation may decide to leave some
bytes to be copied by the epilogue but we can't know that in software,
it's a microarchitecture thing.

There is the case of EL1 debugging itself (kgdb) and it triggers a MOPS
exception to EL2. It would look weird for the guest but I guess the only
other option is to disable MCE2 and let EL1 handle the mismatch MOPS
option itself (assuming it knows how to; it should be fine for Linux). I
think I still prefer Kristina's proposal for KVM as more generic, with
the downside of breaking less usual cases like the kernel
single-stepping itself.

-- 
Catalin

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