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

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Date:   Mon, 18 Feb 2019 16:24:30 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-arm-kernel@...ts.infradead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

On Mon, Feb 18, 2019 at 2:31 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> The question is what "fix it" means. I'm really concerned about AC escapes,
> and everyone else should be, too.

I do think that it might be the right thing to do to add some kind of
WARN_ON_ONCE() for AC being set in various can-reschedule situations.

We'd just have to abstract it sanely. I'm sure arm64 has the exact
same issue with PAN - maybe it saves properly, but the same "we
wouldn't want to go through the scheduler with PAN clear".

On x86, we might as well check DF at the same time as AC.

              Linus