diff mbox

[12/13] powerpc/64: runlatch CTRL[RUN] set optimisation

Message ID 20170613130557.26315-13-npiggin@gmail.com (mailing list archive)
State Rejected
Headers show

Commit Message

Nicholas Piggin June 13, 2017, 1:05 p.m. UTC 
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.

Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kernel/process.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

Comments

Michael Ellerman June 14, 2017, 11:38 a.m. UTC | #1 
Nicholas Piggin <npiggin@gmail.com> writes:

> 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.

Turns out this doesn't work on Cell.

There's an extra field in there:

  Thread enable bits (Read/Write)
  The hypervisor state can suspend its own thread by setting the TE bit
  for its thread to '0’. The hypervisor state can resume the opposite
  thread by setting the TE bit for the opposite thread to '1'. The
  hypervisor state cannot suspend the opposite thread by setting the TE
  bit for the opposite thread to ‘0’. This setting is ignored and does not
  cause an error.
  
  TE0 is the thread enable bit for thread 0.
  TE1 is the thread enable bit for thread 1.
  
  If thread 0 executes the mtctrl instruction, these are the bit values:
  
  [TE0 TE1] Description
    0   0   Disable or suspend thread 0; thread 1 unchanged.
    0   1   Disable or suspend thread 0; enable or resume thread 1 if it was disabled.
    1   0   Unchanged.
    1   1   Enable or resume thread 1 if it was disabled.
  
  If thread 1 executes the mtctrl instruction, these are the bit values:
  
  [TE0 TE1] Description
    0   0    Thread 0 unchanged; disable or suspend thread 1.
    0   1    Unchanged.
    1   0    Enable or resume thread 0 if it was disabled; disable or suspend thread 1.
    1   1    Enable or resume thread 0 if it was disabled.


So writing either 0 or CTRL_RUNLATCH (1) will disable the thread that
does the write - :D

For now I'll just drop this.

cheers
diff mbox

Patch

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
@@ -1960,12 +1960,8 @@  void show_stack(struct task_struct *tsk, unsigned long *stack)
 void notrace __ppc64_runlatch_on(void)
 {
 	struct thread_info *ti = current_thread_info();
-	unsigned long ctrl;
-
-	ctrl = mfspr(SPRN_CTRLF);
-	ctrl |= CTRL_RUNLATCH;
-	mtspr(SPRN_CTRLT, ctrl);
 
+	mtspr(SPRN_CTRLT, CTRL_RUNLATCH);
 	ti->local_flags |= _TLF_RUNLATCH;
 }
 
@@ -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);
 }
 #endif /* CONFIG_PPC64 */