Re: [PATCH v7 09/11] KVM: arm64: guest debug, HW assisted debug support

[Christoffer Dall]

On Mon, Jul 06, 2015 at 09:51:40AM +0100, Will Deacon wrote:
> On Fri, Jul 03, 2015 at 05:07:41PM +0100, Alex Bennée wrote:
> > Will Deacon <will.deacon@xxxxxxx> writes:
> > > On Thu, Jul 02, 2015 at 02:50:33PM +0100, Alex Bennée wrote:
> > >> Are you happy with this?:
> > >
> > > [...]
> > >
> > >> +/**
> > >> + * kvm_arch_dev_ioctl_check_extension
> > >> + *
> > >> + * We currently assume that the number of HW registers is uniform
> > >> + * across all CPUs (see cpuinfo_sanity_check).
> > >> + */
> > >>  int kvm_arch_dev_ioctl_check_extension(long ext)
> > >>  {
> > >>         int r;
> > >> @@ -64,6 +71,12 @@ int kvm_arch_dev_ioctl_check_extension(long ext)
> > >>         case KVM_CAP_ARM_EL1_32BIT:
> > >>                 r = cpu_has_32bit_el1();
> > >>                 break;
> > >> +       case KVM_CAP_GUEST_DEBUG_HW_BPS:
> > >> +               r = hw_breakpoint_slots(TYPE_INST);
> > >> +               break;
> > >> +       case KVM_CAP_GUEST_DEBUG_HW_WPS:
> > >> +               r  = hw_breakpoint_slots(TYPE_DATA);
> > >> +               break;
> > >
> > > Whilst I much prefer this code, it actually adds an unwanted dependency
> > > on PERF_EVENTS that I didn't think about to start with. Sorry to keep
> > > messing you about -- I guess your original patch is the best thing after
> > > all.
> > 
> > Everything looks to be in hw_breakpoint.[ch] which does depend on
> > CONFIG_HAVE_HW_BREAKPOINT which depends on PERF_EVENTS to be built.
> > However the previous code depended on this behaviour as well.
> 
> I think your original approach (of sticking stuff in the header file) works
> regardless of the CONFIG option, no?
> 
> > It would seem weird to enable guest debug using HW debug registers to
> > debug the guest yet not allowing the host kernel to use them? Of course
> > this is the only code they would share as all the magic of guest
> > debugging is already mostly there for dirty guest handling.
> > 
> > I'm not familiar with Kconfig but it looks like this is all part of
> > arm64 defconfig. Are people really going to want to disable PERF_EVENTS
> > but still debug their guests with HW support?
> 
> Then it's your call. I just find the host dependency on perf a bit weird
> to get guest debug working (especially as the dependency is completely
> "fake" because we don't use any perf infrastructure at all).
> 
Agreed, let's see if we can avoid this.

Thanks,
-Christoffer
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