On Mon, May 19 2014 at 10:05:42 am BST, Anup Patel <anup.patel@xxxxxxxxxx> 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).
I don't think we can do less work than this. Or can we?
Thanks,
M.
--
Jazz is not dead. It just smells funny.
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