[PATCH 2/3] powerpc/64s/idle: avoid sync for KVM state when waking from idle

[Vaidyanathan Srinivasan]

* Nicholas Piggin <npiggin at gmail.com> [2017-11-18 00:08:06]:

> When waking from a CPU idle instruction (e.g., nap or stop), the sync
> for ordering the KVM secondary thread state can be avoided if there
> wakeup is coming from a kernel context rather than KVM context.
> 
> This improves performance for ping-pong benchmark with the stop0 idle
> state by 0.46% for 2 threads in the same core, and 1.02% for different
> cores.

Cool, the improvement comes from avoiding the "sync" alone?
 
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> ---
>  arch/powerpc/kernel/idle_book3s.S | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/idle_book3s.S b/arch/powerpc/kernel/idle_book3s.S
> index 2f8364e7b489..07a306173c5a 100644
> --- a/arch/powerpc/kernel/idle_book3s.S
> +++ b/arch/powerpc/kernel/idle_book3s.S
> @@ -532,6 +532,9 @@ ALT_FTR_SECTION_END_IFSET(CPU_FTR_ARCH_300)
>  	mr	r3,r12
> 
>  #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
> +	lbz	r0,HSTATE_HWTHREAD_STATE(r13)
> +	cmpwi	r0,KVM_HWTHREAD_IN_KERNEL
> +	beq	1f
>  	li	r0,KVM_HWTHREAD_IN_KERNEL
>  	stb	r0,HSTATE_HWTHREAD_STATE(r13)
>  	/* Order setting hwthread_state vs. testing hwthread_req */

With this change, we will not check for HSTATE_HWTHREAD_REQ != 0 
condition but unconditionally goto host kernel if
HSTATE_HWTHREAD_STATE == KVM_HWTHREAD_IN_KERNEL at wakeup.

Host is in ST mode and sibling thread got a wakeup event (door bell) to execute
a new vcpu by calling kvm_start_guest, what will HSTATE_HWTHREAD_STATE be?

Just to clarify, what will the flags looks like for

(a) Host cpu sibling thread is offline and need to execute guest
(b) Host cpu sibling thread is idle and need to execute guest

--Vaidy