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

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Date:   Sat, 16 Feb 2019 00:34:14 +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 06:40:34PM +0100, Peter Zijlstra wrote:
> 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.

I misread (I'm forever confused about what way around IF goes), but you
said it right; IF=0 is interrupts disabled and we very much have that in
the middle of context switch.

(just for giggles I set IF for the initial flags value; and it comes
unstuck _real_ quick)

Now, EFLAGS bit 1 is supposedly always 1, but it really doesn't seem to
matter for POPF.

I went through the other flags, and aside from VIP/VIF (I've no clue),
they looks like 0 should be just fine.