x86/fpu: Optimize fpu_copy() some more on lazy switching systems

[deliverable/linux.git] / arch / x86 / kernel / fpu / core.c

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 41ea25a61b5f35ae51df2bb5e827111a55d4073f..edbb5d04a55810b6381d7c1ea84ecb8d35e4edcf 100644 (file)
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -220,16 +220,35 @@ static void fpu_copy(struct fpu *dst_fpu, struct fpu *src_fpu)
 {
        WARN_ON(src_fpu != &current->thread.fpu);
 
-       if (use_eager_fpu()) {
+       /*
+        * Don't let 'init optimized' areas of the XSAVE area
+        * leak into the child task:
+        */
+       if (use_eager_fpu())
                memset(&dst_fpu->state.xsave, 0, xstate_size);
-               copy_fpregs_to_fpstate(dst_fpu);
-       } else {
-               preempt_disable();
-               if (!copy_fpregs_to_fpstate(src_fpu))
-                       fpregs_deactivate(src_fpu);
-               preempt_enable();
-               memcpy(&dst_fpu->state, &src_fpu->state, xstate_size);
+
+       /*
+        * Save current FPU registers directly into the child
+        * FPU context, without any memory-to-memory copying.
+        *
+        * If the FPU context got destroyed in the process (FNSAVE
+        * done on old CPUs) then copy it back into the source
+        * context and mark the current task for lazy restore.
+        *
+        * We have to do all this with preemption disabled,
+        * mostly because of the FNSAVE case, because in that
+        * case we must not allow preemption in the window
+        * between the FNSAVE and us marking the context lazy.
+        *
+        * It shouldn't be an issue as even FNSAVE is plenty
+        * fast in terms of critical section length.
+        */
+       preempt_disable();
+       if (!copy_fpregs_to_fpstate(dst_fpu)) {
+               memcpy(&src_fpu->state, &dst_fpu->state, xstate_size);
+               fpregs_deactivate(src_fpu);
        }
+       preempt_enable();
 }
 
 int fpu__copy(struct fpu *dst_fpu, struct fpu *src_fpu)