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Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190215174034.GP32494@hirez.programming.kicks-ass.net>
Date:   Fri, 15 Feb 2019 10:28:08 -0800
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Brian Gerst <brgerst@gmail.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "linux-alpha@vger.kernel.org" <linux-arm-kernel@lists.infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>,
        "H. Peter Anvin" <hpa@zytor.com>, valentin.schneider@arm.com,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Feb 15, 2019, at 9:40 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
>> 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@infradead.org> w=
rote:
>>>=20
>>> Something like the below, right?
>>>=20
>>> +       frame->flags =3D 0;
>>> +       frame->flags =3D 0;
>>=20
>> Those are not valid flag values.
>>=20
>> Can you popf them? Yes.
>>=20
>> Do they make sense? No.
>>=20
>> It has the IF flag clear, for example. Is that intentional? If it is,
>=20
> 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.

And probably add a comment near the POPF explaining that it will keep IRQs o=
ff :)=