Re: [PATCH v4 2/2] hvf: implement guest debugging on Apple Silicon hosts

[Francesco Cagnin]

> Other than that, the structure of the patch looks OK, but
> I think you need to identify the cause of the problems
> with SMP setups that you mention in the cover letter,
> since they suggest that there's a bug lurking in here
> somewhere.

In the current patch, in `hvf_arch_update_guest_debug()` I'm enabling
exiting the guest on debug exceptions only for the vCPUs that have inserted
software/hardware breakpoint or are singlestepping. In SMP setups this logic
looks flawed, since for example if vCPU #1 sets a software breakpoint and
vCPU #2 hits it, the generated debug exception for vCPU #2 will not exit the
guest and lead to panic for unexpected BRK. A possible fix is enabling
exiting the guest on debug exceptions for all vCPUs (and not just the ones
that have inserted breakpoints)—is this the way to go?

There's also a second analogous issue for which it feels like I'm missing
something. If through GDB a software breakpoint is inserted from vCPU #1 and
later vCPU #2 hits it, then when trying to resume execution after the hit
GDB fails with 'Cannot remove breakpoints', due to
`hvf_find_sw_breakpoint()` returning error because it (correctly) doesn't
find any software breakpoint for vCPU #2 (queue
`cpu->hvf->hvf_sw_breakpoints`). A possible fix seems to be modifying
`hvf_find_sw_breakpoint()` so that it searches for the breakpoint on all
vCPUs' queues; but I've skimmed through the analogous routines for TCG and
KVM and there's nothing resembling this fix, so I wonder why TCG and KVM
don't fail on my example GDB scenario?

With both proposed fixes the patch seems to work well also in SMP setups in
my limited tests.

For the next round, I'll also split the patch as suggested.

Thanks,
Francesco