x86/fpu: Remove fpu->initialized

The struct fpu.initialized member is always set to one for user tasks
and zero for kernel tasks. This avoids saving/restoring the FPU
registers for kernel threads.

The ->initialized = 0 case for user tasks has been removed in previous
changes, for instance, by doing an explicit unconditional init at fork()
time for FPU-less systems which was otherwise delayed until the emulated
opcode.

The context switch code (switch_fpu_prepare() + switch_fpu_finish())
can't unconditionally save/restore registers for kernel threads. Not
only would it slow down the switch but also load a zeroed xcomp_bv for
XSAVES.

For kernel_fpu_begin() (+end) the situation is similar: EFI with runtime
services uses this before alternatives_patched is true. Which means that
this function is used too early and it wasn't the case before.

For those two cases, use current->mm to distinguish between user and
kernel thread. For kernel_fpu_begin() skip save/restore of the FPU
registers.

During the context switch into a kernel thread don't do anything. There
is no reason to save the FPU state of a kernel thread.

The reordering in __switch_to() is important because the current()
pointer needs to be valid before switch_fpu_finish() is invoked so ->mm
is seen of the new task instead the old one.

N.B.: fpu__save() doesn't need to check ->mm because it is called by
user tasks only.

 [ bp: Massage. ]

Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-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: Babu Moger <[email protected]>
Cc: "Chang S. Bae" <[email protected]>
Cc: Dmitry Safonov <[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: Joerg Roedel <[email protected]>
Cc: kvm ML <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Nicolai Stange <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 7888a41a03cd..77d9eb43ccac 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -288,10 +288,10 @@ __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_fpu, cpu);
-
 	this_cpu_write(current_task, next_p);
 
+	switch_fpu_finish(next_fpu, cpu);
+
 	/* Load the Intel cache allocation PQR MSR. */
 	resctrl_sched_in();