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authorLinus Torvalds <torvalds@linux-foundation.org>2012-02-18 20:56:35 (GMT)
committerLinus Torvalds <torvalds@linux-foundation.org>2012-02-18 22:03:48 (GMT)
commit34ddc81a230b15c0e345b6b253049db731499f7e (patch)
tree0c3afd68071ec1a8a1d8724ef9a42ef845ecf402 /arch/x86/kernel/process_64.c
parentf94edacf998516ac9d849f7bc6949a703977a7f3 (diff)
i387: re-introduce FPU state preloading at context switch time
After all the FPU state cleanups and finally finding the problem that caused all our FPU save/restore problems, this re-introduces the preloading of FPU state that was removed in commit b3b0870ef3ff ("i387: do not preload FPU state at task switch time"). However, instead of simply reverting the removal, this reimplements preloading with several fixes, most notably - properly abstracted as a true FPU state switch, rather than as open-coded save and restore with various hacks. In particular, implementing it as a proper FPU state switch allows us to optimize the CR0.TS flag accesses: there is no reason to set the TS bit only to then almost immediately clear it again. CR0 accesses are quite slow and expensive, don't flip the bit back and forth for no good reason. - Make sure that the same model works for both x86-32 and x86-64, so that there are no gratuitous differences between the two due to the way they save and restore segment state differently due to architectural differences that really don't matter to the FPU state. - Avoid exposing the "preload" state to the context switch routines, and in particular allow the concept of lazy state restore: if nothing else has used the FPU in the meantime, and the process is still on the same CPU, we can avoid restoring state from memory entirely, just re-expose the state that is still in the FPU unit. That optimized lazy restore isn't actually implemented here, but the infrastructure is set up for it. Of course, older CPU's that use 'fnsave' to save the state cannot take advantage of this, since the state saving also trashes the state. In other words, there is now an actual _design_ to the FPU state saving, rather than just random historical baggage. Hopefully it's easier to follow as a result. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'arch/x86/kernel/process_64.c')
-rw-r--r--arch/x86/kernel/process_64.c5
1 files changed, 4 insertions, 1 deletions
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 753e803..1fd94bc 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -386,8 +386,9 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
int cpu = smp_processor_id();
struct tss_struct *tss = &per_cpu(init_tss, cpu);
unsigned fsindex, gsindex;
+ fpu_switch_t fpu;
- __unlazy_fpu(prev_p);
+ fpu = switch_fpu_prepare(prev_p, next_p);
/*
* Reload esp0, LDT and the page table pointer:
@@ -457,6 +458,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
prev->gsindex = gsindex;
+ switch_fpu_finish(next_p, fpu);
+
/*
* Switch the PDA and FPU contexts.
*/

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