x86/fpu: Remove user_fpu_begin()

user_fpu_begin() sets fpu_fpregs_owner_ctx to task's fpu struct. This is
always the case since there is no lazy FPU anymore.

fpu_fpregs_owner_ctx is used during context switch to decide if it needs
to load the saved registers or if the currently loaded registers are
valid. It could be skipped during a

  taskA -> kernel thread -> taskA

switch because the switch to the kernel thread would not alter the CPU's
sFPU tate.

Since this field is always updated during context switch and
never invalidated, setting it manually (in user context) makes no
difference. A kernel thread with kernel_fpu_begin() block could
set fpu_fpregs_owner_ctx to NULL but a kernel thread does not use
user_fpu_begin().

This is a leftover from the lazy-FPU time.

Remove user_fpu_begin(), it does not change fpu_fpregs_owner_ctx's
content.

Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>
Reviewed-by: Dave Hansen <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Aubrey Li <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: "Jason A. Donenfeld" <[email protected]>
Cc: kvm ML <[email protected]>
Cc: Nicolai Stange <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

1 files changed, 0 insertions, 1 deletions

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 34989d2a8893..155f4552413e 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -322,7 +322,6 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 		 * For 64-bit frames and 32-bit fsave frames, restore the user
 		 * state to the registers directly (with exceptions handled).
 		 */
-		user_fpu_begin();
 		if (copy_user_to_fpregs_zeroing(buf_fx, xfeatures, fx_only)) {
 			fpu__clear(fpu);
 			return -1;