powerpc/kernel: Drop HMT_MEDIUM_PPR_DISCARD

HMT_MEDIUM_PPR_DISCARD is a macro which is present at the start of most
of our first level exception handlers. It conditionally executes a
HMT_MEDIUM instruction, which sets the processor priority to medium.

On on modern systems, ie. Power7 and later, it is nop'ed out at boot.
All it does is make the exception vectors more cramped, and consume 4
bytes of icache.

On old systems it has the effect of boosting the processor priority at
the start of exception processing. If we were previously in the idle
loop for example, we may be at low or very low priority. This is
desirable as we want to process the exception as fast as possible.

However looking closely at the generated code, we see that in all cases
we execute another HMT_MEDIUM just four instructions later. With code
patching applied, the final code on an old (Power6) system will look
like, eg:

  c000000000000300 <data_access_pSeries>:
  c000000000000300:	7c 42 13 78	mr	r2,r2		<-
  c000000000000304:	7d b2 43 a6	mtsprg	2,r13
  c000000000000308:	7d b1 42 a6	mfsprg	r13,1
  c00000000000030c:	f9 2d 00 80	std	r9,128(r13)
  c000000000000310:	60 00 00 00	nop
  c000000000000314:	7c 42 13 78	mr	r2,r2		<-

So I suggest that the added code complexity of HMT_MEDIUM_PPR_DISCARD is
not justified by the benefit of boosting the processor priority for the
duration of four instructions, and therefore we drop it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 0 insertions, 15 deletions

diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
index 9ee10781121f..60b2bbda212d 100644
--- a/arch/powerpc/include/asm/exception-64s.h
+++ b/arch/powerpc/include/asm/exception-64s.h
@@ -130,15 +130,6 @@ BEGIN_FTR_SECTION_NESTED(941)						\
 END_FTR_SECTION_NESTED(CPU_FTR_HAS_PPR,CPU_FTR_HAS_PPR,941)
 
 /*
- * Increase the priority on systems where PPR save/restore is not
- * implemented/ supported.
- */
-#define HMT_MEDIUM_PPR_DISCARD						\
-BEGIN_FTR_SECTION_NESTED(942)						\
-	HMT_MEDIUM;							\
-END_FTR_SECTION_NESTED(CPU_FTR_HAS_PPR,0,942)  /*non P7*/		
-
-/*
  * Get an SPR into a register if the CPU has the given feature
  */
 #define OPT_GET_SPR(ra, spr, ftr)					\
@@ -346,7 +337,6 @@ do_kvm_##n:								\
 	. = loc;					\
 	.globl label##_pSeries;				\
 label##_pSeries:					\
-	HMT_MEDIUM_PPR_DISCARD;				\
 	SET_SCRATCH0(r13);		/* save r13 */		\
 	EXCEPTION_PROLOG_PSERIES(PACA_EXGEN, label##_common,	\
 				 EXC_STD, KVMTEST, vec)
@@ -362,7 +352,6 @@ label##_pSeries:						\
 	. = loc;					\
 	.globl label##_hv;				\
 label##_hv:						\
-	HMT_MEDIUM_PPR_DISCARD;				\
 	SET_SCRATCH0(r13);	/* save r13 */			\
 	EXCEPTION_PROLOG_PSERIES(PACA_EXGEN, label##_common,	\
 				 EXC_HV, KVMTEST, vec)
@@ -378,7 +367,6 @@ label##_hv:						\
 	. = loc;					\
 	.globl label##_relon_pSeries;			\
 label##_relon_pSeries:					\
-	HMT_MEDIUM_PPR_DISCARD;				\
 	/* No guest interrupts come through here */	\
 	SET_SCRATCH0(r13);		/* save r13 */	\
 	EXCEPTION_RELON_PROLOG_PSERIES(PACA_EXGEN, label##_common, \
@@ -394,7 +382,6 @@ label##_relon_pSeries:						\
 	. = loc;					\
 	.globl label##_relon_hv;			\
 label##_relon_hv:					\
-	HMT_MEDIUM_PPR_DISCARD;				\
 	/* No guest interrupts come through here */	\
 	SET_SCRATCH0(r13);	/* save r13 */		\
 	EXCEPTION_RELON_PROLOG_PSERIES(PACA_EXGEN, label##_common, \
@@ -448,7 +435,6 @@ label##_relon_hv:						\
 	. = loc;							\
 	.globl label##_pSeries;						\
 label##_pSeries:							\
-	HMT_MEDIUM_PPR_DISCARD;						\
 	_MASKABLE_EXCEPTION_PSERIES(vec, label,				\
 				    EXC_STD, SOFTEN_TEST_PR)
 
@@ -466,7 +452,6 @@ label##_hv:								\
 	EXCEPTION_PROLOG_PSERIES_1(label##_common, EXC_HV);
 
 #define __MASKABLE_RELON_EXCEPTION_PSERIES(vec, label, h, extra)	\
-	HMT_MEDIUM_PPR_DISCARD;						\
 	SET_SCRATCH0(r13);    /* save r13 */				\
 	EXCEPTION_PROLOG_0(PACA_EXGEN);					\
 	__EXCEPTION_PROLOG_1(PACA_EXGEN, extra, vec);		\