Supporting KVM_GUESTDBG_BLOCKIRQ or something similar on ARM64

[Marc Zyngier]

On Tue, 29 Oct 2024 08:52:41 +0000,
Ard Biesheuvel <ardb at kernel.org> wrote:
> 
> On Mon, 28 Oct 2024 at 12:23, Marc Zyngier <maz at kernel.org> wrote:
> >
> > > Let's start a discussion about what needs to be done to support this on
> > > arm64.
> >
> > A good start would be to define the semantics of such a flag:
> >
> > - what should it affect? the vcpu you are single-stepping? all vcpu?
> >
> > - should userspace to know that interrupts are pending?
> >
> > - should this result in any effect on the guest's view of time?
> >
> > - what of interactions on the rest of the system (such as devices)?
> >
> 
> Sorry to give a handwavy answer here, but approaching this from a
> usability PoV (like what Puranjay is doing), it is really about
> adhering to the principle of least surprise for the user.
> 
> So in that sense, it is not really about blocking IRQs at all, as long
> as we step over them rather than into them. How that is achieved is
> not that relevant from the user PoV, and maybe KVM_GUESTDBG_BLOCKIRQ
> is not the right solution for ARM at all.

I definitely sympathise with the goal, but there is no simple way to
let interrupts through while stepping (which is what your "step over"
implies):

- the hypervisor (in general) doesn't interact with the guest delivery
  and handling of interrupts -- this is either very opaque (list
  registers) or completely invisible (direct injection)

- replacing the step with a breakpoint after the stepped instruction
  requires us to decode the guest instructions to handle branching
  effects

One possible mechanism would be to:

- while stepping, add breakpoints to the interrupt vectors for the EL
  we are stepping (3 breakpoints for any of the 4 possible exception
  groups),

- when any interrupt breakpoint hits, clear all 3, place a breakpoint
  on the instruction that was about to be single-stepped (pointed to
  by SPSR)

- run to completion, until the breakpoint hits

- disable the breakpoint, reinstall the previous 3 interrupt
  breakpoints

- single-step, rinse, repeat

But then I'm asking myself the question: why is this KVM's job? It
seems to me that this is what an external debugger would do when
interacting with HW on bare metal.

So can we implement this as part of the debugger's state machine?

Thanks,

	M.

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