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

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Message-ID: <alpine.DEB.2.21.1902212231490.1612@nanos.tec.linutronix.de>
Date:   Thu, 21 Feb 2019 22:35:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Julien Thierry <julien.thierry@....com>
cc:     "H. Peter Anvin" <hpa@...or.com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

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.

Thanks,

	tglx