[RFC v2 PATCH 1/1] ARM64: KGDB: Add FP/SIMD debug support

[Ard Biesheuvel]

On 28 February 2014 18:36, Will Deacon <will.deacon at arm.com> wrote:
> On Fri, Feb 28, 2014 at 11:08:58AM +0000, Vijay Kilari wrote:
>> On Fri, Nov 29, 2013 at 3:27 PM, Vijay Kilari <vijay.kilari at gmail.com> wrote:
>> >>> KGDB, being kernel debugger it was thought that allow
>> >>> debugging only if kernel mode is supported.
>> >>> In fact, I proposed (previous email)  to remove this condition and
>> >>> allow to reading neon
>> >>> registers always and allow write only if neon is in kernel mode.
>> >>>
>> >>
>> >> I think KGDB is a special case here and should not considered as a
>> >> user of kernel mode NEON. So even writing the registers should
>> >> allowable regardless of this setting imo.
>> >>
>>
>>  Can we avoid checking on NEON state and allow debugger to
>> access NEON registers irrespective of state to keep it simple?
>> and just compile config is enough?
>
> I'm afraid I don't follow... I think we *do* need to do the relevant
> flush/sync operations, I just don't think it should be predicated on
> KERNEL_MODE_NEON.

Could you elaborate a bit on what you consider flush/sync operations?

The point I was making is that, from kgdb's point of view, it is
irrelevant whether the FPSIMD registers contain current's userland
FPSIMD state or something else. kgdb should return and/or manipulate
what the kernel itself sees when it accesses the FPSIMD registers
directly, regardless of whether those contents belong to current's
userland or not. (please refer to my pending series on kernel mode
NEON optimizations)

CONFIG_KERNEL_MODE_NEON is a 'def_bool y' on arm64, so whether it is
more correct to depend on it or not is not that relevant.

Regards,
Ard.