path: root/arch/x86/kernel/process_64.c
diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2012-02-19 13:27:00 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-02-20 10:58:54 -0800
commit7e16838d94b566a17b65231073d179bc04d590c8 (patch)
tree356ae3999d89d2419fd4b85b062a24820f4a4d82 /arch/x86/kernel/process_64.c
parent80ab6f1e8c981b1b6604b2f22e36c917526235cd (diff)
i387: support lazy restore of FPU state
This makes us recognize when we try to restore FPU state that matches what we already have in the FPU on this CPU, and avoids the restore entirely if so. To do this, we add two new data fields: - a percpu 'fpu_owner_task' variable that gets written any time we update the "has_fpu" field, and thus acts as a kind of back-pointer to the task that owns the CPU. The exception is when we save the FPU state as part of a context switch - if the save can keep the FPU state around, we leave the 'fpu_owner_task' variable pointing at the task whose FP state still remains on the CPU. - a per-thread 'last_cpu' field, that indicates which CPU that thread used its FPU on last. We update this on every context switch (writing an invalid CPU number if the last context switch didn't leave the FPU in a lazily usable state), so we know that *that* thread has done nothing else with the FPU since. These two fields together can be used when next switching back to the task to see if the CPU still matches: if 'fpu_owner_task' matches the task we are switching to, we know that no other task (or kernel FPU usage) touched the FPU on this CPU in the meantime, and if the current CPU number matches the 'last_cpu' field, we know that this thread did no other FP work on any other CPU, so the FPU state on the CPU must match what was saved on last context switch. In that case, we can avoid the 'f[x]rstor' entirely, and just clear the CR0.TS bit. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'arch/x86/kernel/process_64.c')
1 files changed, 1 insertions, 1 deletions
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 8ad880b3bc1c..cfa5c90c01db 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -389,7 +389,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
unsigned fsindex, gsindex;
fpu_switch_t fpu;
- fpu = switch_fpu_prepare(prev_p, next_p);
+ fpu = switch_fpu_prepare(prev_p, next_p, cpu);
* Reload esp0, LDT and the page table pointer:

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