4 files changed, 61 insertions, 33 deletions

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index fb7f0d64e14f..0b20bbb758f2 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -26,6 +26,7 @@ extern void fpu_init(void);
 extern void mxcsr_feature_mask_init(void);
 extern int init_fpu(struct task_struct *child);
 extern asmlinkage void math_state_restore(void);
+extern void __math_state_restore(void);
 extern void init_thread_xstate(void);
 extern int dump_fpu(struct pt_regs *, struct user_i387_struct *);
 
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 59f4524984af..a80eddd41658 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -350,14 +350,21 @@ __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);
+        bool preload_fpu;
 
         /* never put a printk in __switch_to... printk() calls wake_up*() indirectly */
 
-        __unlazy_fpu(prev_p);
+        /*
+         * If the task has used fpu the last 5 timeslices, just do a full
+         * restore of the math state immediately to avoid the trap; the
+         * chances of needing FPU soon are obviously high now
+         */
+        preload_fpu = tsk_used_math(next_p) && next_p->fpu_counter > 5;
 
+        __unlazy_fpu(prev_p);
 
         /* we're going to use this soon, after a few expensive things */
-        if (next_p->fpu_counter > 5)
+        if (preload_fpu)
                 prefetch(next->xstate);
 
         /*
@@ -398,6 +405,11 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
                      task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT))
                 __switch_to_xtra(prev_p, next_p, tss);
 
+        /* If we're going to preload the fpu context, make sure clts
+           is run while we're batching the cpu state updates. */
+        if (preload_fpu)
+                clts();
+
         /*
          * Leave lazy mode, flushing any hypercalls made here.
          * This must be done before restoring TLS segments so
@@ -407,15 +419,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
          */
         arch_end_context_switch(next_p);
 
-        /* If the task has used fpu the last 5 timeslices, just do a full
-         * restore of the math state immediately to avoid the trap; the
-         * chances of needing FPU soon are obviously high now
-         *
-         * tsk_used_math() checks prevent calling math_state_restore(),
-         * which can sleep in the case of !tsk_used_math()
-         */
-        if (tsk_used_math(next_p) && next_p->fpu_counter > 5)
-                math_state_restore();
+        if (preload_fpu)
+                __math_state_restore();
 
         /*
          * Restore %gs if needed (which is common)
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index ebefb5407b9d..a28279dbb07c 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -386,9 +386,17 @@ __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;
+        bool preload_fpu;
+
+        /*
+         * If the task has used fpu the last 5 timeslices, just do a full
+         * restore of the math state immediately to avoid the trap; the
+         * chances of needing FPU soon are obviously high now
+         */
+        preload_fpu = tsk_used_math(next_p) && next_p->fpu_counter > 5;
 
         /* we're going to use this soon, after a few expensive things */
-        if (next_p->fpu_counter > 5)
+        if (preload_fpu)
                 prefetch(next->xstate);
 
         /*
@@ -419,6 +427,13 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 
         load_TLS(next, cpu);
 
+        /* Must be after DS reload */
+        unlazy_fpu(prev_p);
+
+        /* Make sure cpu is ready for new context */
+        if (preload_fpu)
+                clts();
+
         /*
          * Leave lazy mode, flushing any hypercalls made here.
          * This must be done before restoring TLS segments so
@@ -459,9 +474,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
                 wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
         prev->gsindex = gsindex;
 
-        /* Must be after DS reload */
-        unlazy_fpu(prev_p);
-
         /*
          * Switch the PDA and FPU contexts.
          */
@@ -480,15 +492,12 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
                      task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV))
                 __switch_to_xtra(prev_p, next_p, tss);
 
-        /* If the task has used fpu the last 5 timeslices, just do a full
-         * restore of the math state immediately to avoid the trap; the
-         * chances of needing FPU soon are obviously high now
-         *
-         * tsk_used_math() checks prevent calling math_state_restore(),
-         * which can sleep in the case of !tsk_used_math()
+        /*
+         * Preload the FPU context, now that we've determined that the
+         * task is likely to be using it. 
          */
-        if (tsk_used_math(next_p) && next_p->fpu_counter > 5)
-                math_state_restore();
+        if (preload_fpu)
+                __math_state_restore();
         return prev_p;
 }
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 6fe85c272a2b..83264922a878 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -795,6 +795,28 @@ asmlinkage void __attribute__((weak)) smp_threshold_interrupt(void)
 }
 
 /*
+ * __math_state_restore assumes that cr0.TS is already clear and the
+ * fpu state is all ready for use.  Used during context switch.
+ */
+void __math_state_restore(void)
+{
+        struct thread_info *thread = current_thread_info();
+        struct task_struct *tsk = thread->task;
+
+        /*
+         * Paranoid restore. send a SIGSEGV if we fail to restore the state.
+         */
+        if (unlikely(restore_fpu_checking(tsk))) {
+                stts();
+                force_sig(SIGSEGV, tsk);
+                return;
+        }
+
+        thread->status |= TS_USEDFPU;   /* So we fnsave on switch_to() */
+        tsk->fpu_counter++;
+}
+
+/*
  * 'math_state_restore()' saves the current math information in the
  * old math state array, and gets the new ones from the current task
  *
@@ -825,17 +847,8 @@ asmlinkage void math_state_restore(void)
         }
 
         clts();                         /* Allow maths ops (or we recurse) */
-        /*
-         * Paranoid restore. send a SIGSEGV if we fail to restore the state.
-         */
-        if (unlikely(restore_fpu_checking(tsk))) {
-                stts();
-                force_sig(SIGSEGV, tsk);
-                return;
-        }
 
-        thread->status |= TS_USEDFPU;   /* So we fnsave on switch_to() */
-        tsk->fpu_counter++;
+        __math_state_restore();
 }
 EXPORT_SYMBOL_GPL(math_state_restore);