sched/x86: Save [ER]FLAGS on context switch

Effectively reverts commit:

  2c7577a75837 ("sched/x86_64: Don't save flags on context switch")

Specifically because SMAP uses FLAGS.AC which invalidates the claim
that the kernel has clean flags.

In particular; while preemption from interrupt return is fine (the
IRET frame on the exception stack contains FLAGS) it breaks any code
that does synchonous scheduling, including preempt_enable().

This has become a significant issue ever since commit:

  5b24a7a2aa20 ("Add 'unsafe' user access functions for batched accesses")

provided for means of having 'normal' C code between STAC / CLAC,
exposing the FLAGS.AC state. So far this hasn't led to trouble,
however fix it before it comes apart.

Reported-by: Julien Thierry <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Fixes: 5b24a7a2aa20 ("Add 'unsafe' user access functions for batched accesses")
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions