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

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Message-ID: <20190215174034.GP32494@hirez.programming.kicks-ass.net>
Date:   Fri, 15 Feb 2019 18:40:34 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Brian Gerst <brgerst@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <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>,
        "H. Peter Anvin" <hpa@...or.com>, valentin.schneider@....com,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy 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 Fri, Feb 15, 2019 at 09:18:00AM -0800, Linus Torvalds wrote:
> On Thu, Feb 14, 2019 at 11:34 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Something like the below, right?
> >
> > +       frame->flags = 0;
> > +       frame->flags = 0;
> 
> Those are not valid flag values.
> 
> Can you popf them? Yes.
> 
> Do they make sense? No.
> 
> It has the IF flag clear, for example. Is that intentional? If it is,

Uhmm. yeah, that's bonkers. We should have interrupts disabled here.
I'll go read up on the eflags and figure out what they _should_ be right
about there.