x86/fpu: Micro-optimize the copy_xregs_to_kernel*() and copy_kernel_to_xregs*() functions

The copy_xregs_to_kernel*() and copy_kernel_to_xregs*() functions are used
to copy FPU registers to kernel memory and vice versa.

They are never expected to fail, yet they have a return code, mostly because
that way they can share the assembly macros with the copy*user*() functions.

This error code is then silently ignored by the context switching
and other code - which made the bug in:

  b8c1b8ea7b21 ("x86/fpu: Fix FPU state save area alignment bug")

harder to fix than necessary.

So remove the return values and check for no faults when FPU debugging
is enabled in the .config.

This improves the eagerfpu context switching fast path by a couple of
instructions, when FPU debugging is disabled:

   ffffffff810407fa:      89 c2                   mov    %eax,%edx
   ffffffff810407fc:      48 0f ae 2f             xrstor64 (%rdi)
   ffffffff81040800:      31 c0                   xor    %eax,%eax
  -ffffffff81040802:      eb 0a                   jmp    ffffffff8104080e <__switch_to+0x321>
  +ffffffff81040802:      eb 16                   jmp    ffffffff8104081a <__switch_to+0x32d>
   ffffffff81040804:      31 c0                   xor    %eax,%eax
   ffffffff81040806:      48 0f ae 8b c0 05 00    fxrstor64 0x5c0(%rbx)
   ffffffff8104080d:      00

Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions