Re: [PATCH v2 2/3] powerpc/kvm/book3s_hv: Enable CPUs to run guest after waking up from fast-sleep

On 10/07/2014 10:41 AM, Benjamin Herrenschmidt wrote:

On Wed, 2014-10-01 at 13:15 +0530, Shreyas B. Prabhu wrote:

quoted

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 050f79a..c64f3cc0 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S

@@ -100,25 +100,8 @@ system_reset_pSeries:
 	SET_SCRATCH0(r13)
 #ifdef CONFIG_PPC_P7_NAP
 BEGIN_FTR_SECTION
-	/* Running native on arch 2.06 or later, check if we are
-	 * waking up from nap. We only handle no state loss and
-	 * supervisor state loss. We do -not- handle hypervisor
-	 * state loss at this time.
-	 */
-	mfspr	r13,SPRN_SRR1
-	rlwinm.	r13,r13,47-31,30,31
-	beq	9f
 
-	/* waking up from powersave (nap) state */
-	cmpwi	cr1,r13,2
-	/* Total loss of HV state is fatal, we could try to use the
-	 * PIR to locate a PACA, then use an emergency stack etc...
-	 * OPAL v3 based powernv platforms have new idle states
-	 * which fall in this catagory.
-	 */
-	bgt	cr1,8f
 	GET_PACA(r13)
-
 #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
 	li	r0,KVM_HWTHREAD_IN_KERNEL
 	stb	r0,HSTATE_HWTHREAD_STATE(r13)

@@ -131,13 +114,27 @@ BEGIN_FTR_SECTION
 1:
 #endif

So you moved the state loss check to after the KVM check ? Was this
reviewed by Paul ? Is that ok ? (Does this match what we have in
PowerKVM ?). Is it possible that we end up calling kvm_start_guest
after a HV state loss or do we know for sure that this won't happen
for a reason or another ? If that's the case, then that reason needs
to be clearly documented here in a comment.

This wont happen because the first thread in the core which comes out of
an idle state which has a state loss will not enter into KVM since the
HSTATE_HWTHREAD_STATE is not yet set. It continues on to restore the
lost state.

This thread sets the HSTATE_HWTHREAD_STATE and wakes up the remaining
threads in the core. These sibling threads enter kvm directly not
requiring to restore lost state since the first thread has restored it
anyway. So we are safe. We will certainly add a comment there.

Thanks

Regards
Preeti U Murthy