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

On Mon, May 19 2014 at 10:35:58 am BST, Anup Patel [off-list ref] wrote:

On 19 May 2014 14:58, Marc Zyngier [off-list ref] wrote:

quoted

On Mon, May 19 2014 at 10:05:42 am BST, Anup Patel
[off-list ref] wrote

Hi Anup,

quoted

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?

Right. There is already some comments to that effect just above, where
we compute the dirty state, but I think it doesn't hurt to repeat it.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.