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i387: re-introduce FPU state preloading at context switch time

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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 b3b0870ef3 ("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>
This commit is contained in:
Linus Torvalds
2012-02-18 12:56:35 -08:00
parent f94edacf99
commit 34ddc81a23
4 changed files with 139 additions and 48 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
arch/x86/kernel/process_32.c
View File

@@ -299,10 +299,11 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
*next = &next_p->thread;
int cpu = smp_processor_id();
struct tss_struct *tss = &per_cpu(init_tss, cpu);
fpu_switch_t fpu;
/* never put a printk in __switch_to... printk() calls wake_up*() indirectly */
__unlazy_fpu(prev_p);
fpu = switch_fpu_prepare(prev_p, next_p);
/*
* Reload esp0.
@@ -357,6 +358,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
if (prev->gs | next->gs)
lazy_load_gs(next->gs);
switch_fpu_finish(next_p, fpu);
percpu_write(current_task, next_p);
return prev_p;

5
arch/x86/kernel/process_64.c
View File

@@ -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.
*/

67
arch/x86/kernel/traps.c
View File

@@ -571,40 +571,16 @@ asmlinkage void __attribute__((weak)) smp_threshold_interrupt(void)
}
/*
* 'math_state_restore()' saves the current math information in the
* old math state array, and gets the new ones from the current task
*
* Careful.. There are problems with IBM-designed IRQ13 behaviour.
* Don't touch unless you *really* know how it works.
*
* Must be called with kernel preemption disabled (eg with local
* local interrupts as in the case of do_device_not_available).
* This gets called with the process already owning the
* FPU state, and with CR0.TS cleared. It just needs to
* restore the FPU register state.
*/
void math_state_restore(void)
void __math_state_restore(struct task_struct *tsk)
{
struct task_struct *tsk = current;
/* We need a safe address that is cheap to find and that is already
in L1. We're just bringing in "tsk->thread.has_fpu", so use that */
in L1. We've just brought in "tsk->thread.has_fpu", so use that */
#define safe_address (tsk->thread.has_fpu)
if (!tsk_used_math(tsk)) {
local_irq_enable();
/*
* does a slab alloc which can sleep
*/
if (init_fpu(tsk)) {
/*
* ran out of memory!
*/
do_group_exit(SIGKILL);
return;
}
local_irq_disable();
}
__thread_fpu_begin(tsk);
/* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
is pending. Clear the x87 state here by setting it to fixed
values. safe_address is a random variable that should be in L1 */
@@ -623,6 +599,39 @@ void math_state_restore(void)
force_sig(SIGSEGV, tsk);
return;
}
}
/*
* 'math_state_restore()' saves the current math information in the
* old math state array, and gets the new ones from the current task
*
* Careful.. There are problems with IBM-designed IRQ13 behaviour.
* Don't touch unless you *really* know how it works.
*
* Must be called with kernel preemption disabled (eg with local
* local interrupts as in the case of do_device_not_available).
*/
void math_state_restore(void)
{
struct task_struct *tsk = current;
if (!tsk_used_math(tsk)) {
local_irq_enable();
/*
* does a slab alloc which can sleep
*/
if (init_fpu(tsk)) {
/*
* ran out of memory!
*/
do_group_exit(SIGKILL);
return;
}
local_irq_disable();
}
__thread_fpu_begin(tsk);
__math_state_restore(tsk);
tsk->fpu_counter++;
}