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

["Alex Bennée" <alex.bennee@linaro.org>]

Will Deacon <will.deacon@arm.com> writes:

> On Fri, Jul 03, 2015 at 05:07:41PM +0100, Alex Bennée wrote:
>> Will Deacon <will.deacon@arm.com> 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?

Ahh yeah I reverted that to an extern due to random compile breakage:

http://storage.kernelci.org/alex/v4.1-12-gd38574dba3ec/arm64-allmodconfig/build.log

I'll see if I can fix that up.

>> 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).
>
> Will

-- 
Alex Bennée