[PATCH 0/9] arm64: KVM: debug infrastructure support

[Anup Patel]

On 19 May 2014 14:58, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On Mon, May 19 2014 at 10:05:42 am BST, Anup Patel <anup.patel at linaro.org> wrote
>
> Hi Anup,
>
>> Overall the patchset looks good to me.
>>
>> The debug register usage by Guest will be very rare
>> so a lazy save/restore makes lot-of-sense here.
>>
>> The only concern here is that amount of time spend in
>> world-switch will increase for Guest once Guest starts
>> accessing debug registers.
>>
>> I was wondering if it is possible to detect that Guest
>> has stopped using debug HW and we can mark debug
>> state as clean. (or something similar)
>
> If you look carefully at patch #8 (last hunk of the patch), you'll see
> that I always reset the debug state to "clean" at the end of a guest
> run:
>
> @@ -609,6 +1040,12 @@ __kvm_vcpu_return:
>
>         bl __restore_sysregs
>         bl __restore_fpsimd
> +
> +       skip_clean_debug_state x3, 1f
> +       // Clear the dirty flag for the next run
> +       str     xzr, [x0, #VCPU_DEBUG_FLAGS]
> +       bl      __restore_debug
> +1:
>         restore_host_regs
>
>         mov     x0, x1
>
> This ensures that the guest's debug state will only be reloaded if:
>
> - MDSCR_EL1 has either MDE or KDE set (which means the guest is actively
> using the debug infrastructure)
> - or the guest has written to a trapped register (which marks the state
> as dirty).

Thanks for pointing out.

Can you add this info as comment in patch#8 where you
clear the dirty flag?

>
> I don't think we can do less work than this. Or can we?
>
> Thanks,
>
>         M.
> --
> Jazz is not dead. It just smells funny.

--
Anup