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, 9 deletions

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 3419cbf2ad59..0757f23d35aa 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -96,7 +96,6 @@ __start_interrupts:
 
 	.globl system_reset_pSeries;
 system_reset_pSeries:
-	HMT_MEDIUM_PPR_DISCARD
 	SET_SCRATCH0(r13)
 #ifdef CONFIG_PPC_P7_NAP
 BEGIN_FTR_SECTION
@@ -164,7 +163,6 @@ machine_check_pSeries_1:
 	 * some code path might still want to branch into the original
 	 * vector
 	 */
-	HMT_MEDIUM_PPR_DISCARD
 	SET_SCRATCH0(r13)		/* save r13 */
 #ifdef CONFIG_PPC_P7_NAP
 BEGIN_FTR_SECTION
@@ -199,7 +197,6 @@ ALT_FTR_SECTION_END_IFSET(CPU_FTR_HVMODE)
 	. = 0x300
 	.globl data_access_pSeries
 data_access_pSeries:
-	HMT_MEDIUM_PPR_DISCARD
 	SET_SCRATCH0(r13)
 	EXCEPTION_PROLOG_PSERIES(PACA_EXGEN, data_access_common, EXC_STD,
 				 KVMTEST, 0x300)
@@ -207,7 +204,6 @@ data_access_pSeries:
 	. = 0x380
 	.globl data_access_slb_pSeries
 data_access_slb_pSeries:
-	HMT_MEDIUM_PPR_DISCARD
 	SET_SCRATCH0(r13)
 	EXCEPTION_PROLOG_0(PACA_EXSLB)
 	EXCEPTION_PROLOG_1(PACA_EXSLB, KVMTEST, 0x380)
@@ -239,7 +235,6 @@ data_access_slb_pSeries:
 	. = 0x480
 	.globl instruction_access_slb_pSeries
 instruction_access_slb_pSeries:
-	HMT_MEDIUM_PPR_DISCARD
 	SET_SCRATCH0(r13)
 	EXCEPTION_PROLOG_0(PACA_EXSLB)
 	EXCEPTION_PROLOG_1(PACA_EXSLB, KVMTEST, 0x480)
@@ -269,7 +264,6 @@ instruction_access_slb_pSeries:
 	.globl hardware_interrupt_hv;
 hardware_interrupt_pSeries:
 hardware_interrupt_hv:
-	HMT_MEDIUM_PPR_DISCARD
 	BEGIN_FTR_SECTION
 		_MASKABLE_EXCEPTION_PSERIES(0x502, hardware_interrupt,
 					    EXC_HV, SOFTEN_TEST_HV)
@@ -413,7 +407,6 @@ hv_facility_unavailable_trampoline:
 	. = 0x1500
 	.global denorm_exception_hv
 denorm_exception_hv:
-	HMT_MEDIUM_PPR_DISCARD
 	mtspr	SPRN_SPRG_HSCRATCH0,r13
 	EXCEPTION_PROLOG_0(PACA_EXGEN)
 	EXCEPTION_PROLOG_1(PACA_EXGEN, NOTEST, 0x1500)
@@ -527,7 +520,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HVMODE)
 machine_check_pSeries:
 	.globl machine_check_fwnmi
 machine_check_fwnmi:
-	HMT_MEDIUM_PPR_DISCARD
 	SET_SCRATCH0(r13)		/* save r13 */
 	EXCEPTION_PROLOG_0(PACA_EXMC)
 machine_check_pSeries_0:
@@ -711,7 +703,6 @@ ALT_FTR_SECTION_END_IFSET(CPU_FTR_HVMODE)
 	.globl system_reset_fwnmi
       .align 7
 system_reset_fwnmi:
-	HMT_MEDIUM_PPR_DISCARD
 	SET_SCRATCH0(r13)		/* save r13 */
 	EXCEPTION_PROLOG_PSERIES(PACA_EXGEN, system_reset_common, EXC_STD,
 				 NOTEST, 0x100)