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

From: Thomas Gleixner
Date: Thu Feb 21 2019 - 16:35:57 EST

On Thu, 21 Feb 2019, Julien Thierry wrote:
> On 20/02/2019 22:55, H. Peter Anvin wrote:
> > Now, __fentry__ is kind of a special beast; in some ways it is an "exception
> > implemented as a function call"; on x86 one could even consider using an INT
> > instruction in order to reduce the NOP footprint in the unarmed case. Nor is
> > __fentry__ a C function; it has far more of an exception-like ABI.
> > *Regardless* of what else we do, I believe __fentry__ ought to
> > save/disable/restore AC, just like an exception does.
> >
> That does make sense to me. However it doesn't solve the issue of
> calling (or preventing to call) some function that rescheds.

IMNSHO any call inside a AC region is a bug lurking round the corner. The
only thing which is tolerable is an exception of some sort.

Enforce that with objtool. End of story.