linux-kernel - Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same user process

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Date:   Thu, 25 Jan 2018 14:01:27 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>
Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same
 user process

On Thu, Jan 25, 2018 at 1:57 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> Andy Lutomirski <luto@...nel.org> writes:
>>
>> That being said, just stashing last_user_mm without any refcounting
>> should be fine.
>
> If last_user_mm is freed and reallocated by a different process,
> then that would miss the IPBP incorrectly.
>

Hmm, right.  So ctx_id it is.

--Andy