Re: [PATCH 13/14] powerpc/64: runlatch CTRL[RUN] set optimisation

Vaidyanathan Srinivasan [off-list ref] writes:

* Nicholas Piggin [off-list ref] [2017-06-12 09:58:34]:

quoted

The CTRL register is read-only except bit 63 which is the run latch
control. This means it can be updated with a mtspr rather than
mfspr/mtspr.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

quoted

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index baae104b16c7..a44ea034c226 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c

@@ -1973,13 +1969,9 @@ void notrace __ppc64_runlatch_on(void)
 void notrace __ppc64_runlatch_off(void)
 {
 	struct thread_info *ti = current_thread_info();
-	unsigned long ctrl;

 	ti->local_flags &= ~_TLF_RUNLATCH;
-
-	ctrl = mfspr(SPRN_CTRLF);
-	ctrl &= ~CTRL_RUNLATCH;
-	mtspr(SPRN_CTRLT, ctrl);
+	mtspr(SPRN_CTRLT, 0);

Good idea.  Writing to CTRL register can change only the RUN field.
Was this any different in older generations?

No AFAICS back to 2.02.

Anton and Ben kept the mfspr/mtspr part in earlier updates to this
routine.

Doing the read/modify write is forward compatible vs a new writable
field, whereas writing the whole register with a known value is not.

cheers