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

[Peter Maydell <peter.maydell@linaro.org>]

On Sun, 30 Apr 2023 at 20:45, Francesco Cagnin
<francesco.cagnin@gmail.com> wrote:
>
> > 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?

Yes, I think that makes sense. We treat breakpoints as
VM-wide (and writing a brk instruction into guest memory is
by definition VM-wide), so we should put all the vCPUs into
"we're debugging the VM mode", not just one.

> 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?

For KVM the sw breakpoint list is in cpu->kvm_state, and
we share a single kvm_state across every vCPU in the VM
(kvm_init_vcpu() initializes cpu->kvm_state to be a pointer
to the singleton global kvm_state).
Isn't cpu->hvf shared between vCPUs in the same way?

For TCG breakpoints are effectively always hw breakpoints,
regardless of whether gdb asks for a sw bp or a hw bp;
the code path is different from how KVM and HVF do things.

thanks
-- PMM