From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752615AbdHPRQJ (ORCPT <rfc822;w@1wt.eu>);
        Wed, 16 Aug 2017 13:16:09 -0400
Received: from foss.arm.com ([217.140.101.70]:38302 "EHLO foss.arm.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1752581AbdHPRQG (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 16 Aug 2017 13:16:06 -0400
Date: Tue, 15 Aug 2017 23:35:41 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>,
        Will Deacon <will.deacon@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        joeyli <jlee@suse.com>, Borislav Petkov <bp@alien8.de>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        "Neri, Ricardo" <ricardo.neri@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>
Subject: Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually
 twiddling with cr3
Message-ID: <20170815223541.GA25778@remoulade>
References: <1502824706-30762-1-git-send-email-sai.praneeth.prakhya@intel.com>
 <1502824706-30762-4-git-send-email-sai.praneeth.prakhya@intel.com>
 <CALCETrUnT0YL_F3mXyJzi=NE5jvFUUQbwjdK5224zDKkTts-UQ@mail.gmail.com>
 <CAKv+Gu8AySQSXtRWfHA4y5DbH-DQ7jpqkp=tS+snSTar_sKBJw@mail.gmail.com>
 <20170816095338.GB17270@leverpostej>
 <20170816100709.GG12845@arm.com>
 <20170816110321.GC17270@leverpostej>
 <20170816125715.GB3384@codeblueprint.co.uk>
 <CALCETrVSHuz=hcAgGxYHBmpiNcP4ECrJh885cR4X17QsJVj8+w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CALCETrVSHuz=hcAgGxYHBmpiNcP4ECrJh885cR4X17QsJVj8+w@mail.gmail.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Aug 16, 2017 at 09:14:41AM -0700, Andy Lutomirski wrote:
> On Wed, Aug 16, 2017 at 5:57 AM, Matt Fleming <matt@codeblueprint.co.uk> wrote:
> > On Wed, 16 Aug, at 12:03:22PM, Mark Rutland wrote:
> >>
> >> I'd expect we'd abort at a higher level, not taking any sample. i.e.
> >> we'd have the core overflow handler check in_funny_mm(), and if so, skip
> >> the sample, as with the skid case.
> >
> > FYI, this is my preferred solution for x86 too.
> 
> One option for the "funny mm" flag would be literally the condition
> current->mm != current->active_mm.  I *think* this gets all the cases
> right as long as efi_switch_mm is careful with its ordering and that
> the arch switch_mm() code can handle the resulting ordering.  (x86's
> can now, I think, or at least will be able to in 4.14 -- not sure
> about other arches).

For arm64 we'd have to rework things a bit to get the ordering right
(especially when we flip to/from the idmap), but otherwise this sounds sane to
me.

> That being said, there's a totally different solution: run EFI
> callbacks in a kernel thread.  This has other benefits: we could run
> those callbacks in user mode some day, and doing *that* in a user
> thread seems like a mistake.

I think that wouldn't work for CPU-bound perf events (which are not
ctx-switched with the task).

It might be desireable to do that anyway, though.

Thanks,
Mark.