[PATCH] powerpc/booke: Eliminate rfi from exception entry path.

Unlike classic, we don't really need the MSR change to be atomic with the
branch.  This eliminates a trap as a KVM guest (in the absence of
hardware hypervisor extensions), where mtmsr is paravirtualized but rfi
is not.  For a virtualized guest without any paravirtualization, this
eliminates an additional two traps (SRR0/1).

Signed-off-by: Scott Wood <redacted>
---
 arch/powerpc/kernel/entry_32.S |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index ba3aeb4..6bb637c 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S

@@ -193,6 +193,9 @@ transfer_to_handler_cont:
 	lwz	r11,0(r9)		/* virtual address of handler */
 	lwz	r9,4(r9)		/* where to go when done */
 #ifdef CONFIG_TRACE_IRQFLAGS
+#ifdef CONFIG_BOOKE
+	mtmsr	r10
+#else
 	lis	r12,reenable_mmu@h
 	ori	r12,r12,reenable_mmu@l
 	mtspr	SPRN_SRR0,r12

@@ -201,6 +204,7 @@ transfer_to_handler_cont:
 	RFI
 reenable_mmu:				/* re-enable mmu so we can */
 	mfmsr	r10
+#endif /* !CONFIG_BOOKE */
 	lwz	r12,_MSR(r1)
 	xor	r10,r10,r12
 	andi.	r10,r10,MSR_EE		/* Did EE change? */

@@ -247,11 +251,23 @@ reenable_mmu:				/* re-enable mmu so we can */
 	mtlr	r9
 	bctr				/* jump to handler */
 #else /* CONFIG_TRACE_IRQFLAGS */
+#ifdef CONFIG_BOOKE
+	/*
+	 * We're not changing address space on Book E, and the extra rfi
+	 * can hurt when virtualized without hardware support -- whereas
+	 * mtmsr can be paravirtualized.
+	 */
+	mtmsr	r10
+	mtctr	r11
+	mtlr	r9
+	bctr
+#else
 	mtspr	SPRN_SRR0,r11
 	mtspr	SPRN_SRR1,r10
 	mtlr	r9
 	SYNC
 	RFI				/* jump to handler, enable MMU */
+#endif /* !CONFIG_BOOKE */
 #endif /* CONFIG_TRACE_IRQFLAGS */
 
 #if defined (CONFIG_6xx) || defined(CONFIG_E500)

-- 
1.7.5.4

On Tue, 2012-07-10 at 19:34 -0500, Scott Wood wrote:

Unlike classic, we don't really need the MSR change to be atomic with the
branch.  This eliminates a trap as a KVM guest (in the absence of
hardware hypervisor extensions), where mtmsr is paravirtualized but rfi
is not.  For a virtualized guest without any paravirtualization, this
eliminates an additional two traps (SRR0/1).

In fact, I wonder, what do we write into the MSR at this point that
wasn't already in it in BookE ? RI ? I wonder if we could get away
without the mtmsr alltogether...

Cheers,
Ben.

On 07/10/2012 07:36 PM, Benjamin Herrenschmidt wrote:

On Tue, 2012-07-10 at 19:34 -0500, Scott Wood wrote:

quoted

Unlike classic, we don't really need the MSR change to be atomic with the
branch.  This eliminates a trap as a KVM guest (in the absence of
hardware hypervisor extensions), where mtmsr is paravirtualized but rfi
is not.  For a virtualized guest without any paravirtualization, this
eliminates an additional two traps (SRR0/1).

In fact, I wonder, what do we write into the MSR at this point that
wasn't already in it in BookE ? RI ? I wonder if we could get away
without the mtmsr alltogether...

Doesn't EE get set there for some exceptions?

-Scott

On 11.07.2012, at 02:34, Scott Wood wrote:

Unlike classic, we don't really need the MSR change to be atomic with =

the

branch.  This eliminates a trap as a KVM guest (in the absence of
hardware hypervisor extensions), where mtmsr is paravirtualized but =

rfi

b/arch/powerpc/kernel/entry_32.S

re-enable mmu so we can */

	mtlr	r9
	bctr				/* jump to handler */
#else /* CONFIG_TRACE_IRQFLAGS */
+#ifdef CONFIG_BOOKE
+	/*
+	 * We're not changing address space on Book E, and the extra rfi
+	 * can hurt when virtualized without hardware support -- whereas
+	 * mtmsr can be paravirtualized.

We can always paravirtualize RFI as well if it makes sense.


Alex

On 07/10/2012 07:44 PM, Alexander Graf wrote:

On 11.07.2012, at 02:34, Scott Wood wrote:

quoted

+#ifdef CONFIG_BOOKE
+	/*
+	 * We're not changing address space on Book E, and the extra rfi
+	 * can hurt when virtualized without hardware support -- whereas
+	 * mtmsr can be paravirtualized.

We can always paravirtualize RFI as well if it makes sense.

I'm not sure that's possible.  We thought about it a while back, but
IIRC the difficulty was not leaving a register clobbered.

-Scott

On Tue, 2012-07-10 at 19:41 -0500, Scott Wood wrote:

On 07/10/2012 07:36 PM, Benjamin Herrenschmidt wrote:

quoted

On Tue, 2012-07-10 at 19:34 -0500, Scott Wood wrote:

quoted

Unlike classic, we don't really need the MSR change to be atomic with the
branch.  This eliminates a trap as a KVM guest (in the absence of
hardware hypervisor extensions), where mtmsr is paravirtualized but rfi
is not.  For a virtualized guest without any paravirtualization, this
eliminates an additional two traps (SRR0/1).

In fact, I wonder, what do we write into the MSR at this point that
wasn't already in it in BookE ? RI ? I wonder if we could get away
without the mtmsr alltogether...

Doesn't EE get set there for some exceptions?

It does, tho arguably it shouldn't in most cases :-) I'm happy to turn a
bunch of these into explicit local_irq_enable() in the C code though
which will turn into a wrteei which is more efficient on BookE.

Cheers,
Ben.

On Tue, 2012-07-10 at 19:47 -0500, Scott Wood wrote:

On 07/10/2012 07:44 PM, Alexander Graf wrote:

quoted

On 11.07.2012, at 02:34, Scott Wood wrote:

quoted

+#ifdef CONFIG_BOOKE
+	/*
+	 * We're not changing address space on Book E, and the extra rfi
+	 * can hurt when virtualized without hardware support -- whereas
+	 * mtmsr can be paravirtualized.

We can always paravirtualize RFI as well if it makes sense.

I'm not sure that's possible.  We thought about it a while back, but
IIRC the difficulty was not leaving a register clobbered.

Besides mtmsr is slow on real HW as well. Also paravirt as done today
for complex instructions like mtmsr is racy :-) (I already had a chat
about that with Alex a while back, we might want to re-consider what
kind of fix can be done at some point).

Cheers,
Ben.