linux-kernel - Re: [RFC PATCH v7 17/23] kernel/entry: Add support for core-wide protection of kernel-mode

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Date:   Wed, 02 Sep 2020 18:57:17 +0200
From:   Dario Faggioli <dfaggioli@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Chris Hyser <chris.hyser@...cle.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        mingo@...nel.org, pjt@...gle.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, rostedt@...dmis.org,
        derkling@...gle.com, benbjiang@...cent.com,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Tim Chen <tim.c.chen@...el.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [RFC PATCH v7 17/23] kernel/entry: Add support for core-wide
 protection of kernel-mode

On Wed, 2020-09-02 at 09:53 +0200, Thomas Gleixner wrote:
> On Tue, Sep 01 2020 at 21:29, Joel Fernandes wrote:
> > On Tue, Sep 01, 2020 at 10:02:10PM +0200, Thomas Gleixner wrote:
> > > 
> > Or, are you saying users may want 'core scheduling' enabled but may
> > want to
> > leave out the kernel protection?
> 
> Core scheduling per se without all the protection muck, i.e. a
> relaxed
> version which tries to gang schedule threads of a process on a core
> if
> feasible has advantages to some workloads.
> 
Indeed! For at least two reasons, IMO:

1) what Thomas is saying already. I.e., even on a CPU which has HT but
is not affected by any of the (known!) speculation issues, one may want
to use Core Scheduling _as_a_feature_. For instance, for avoiding
threads from different processes, or vCPUs from different VMs, sharing
cores (e.g., for better managing their behavior/performance, or for
improved fairness of billing/accounting). And in this case, this
mechanism for protecting the kernel from the userspace on the other
thread may not be necessary or interesting;

2) protection of the kernel from the other thread running in userspace
may be achieved in different ways. This is one, sure. ASI will probably
be another. Hence if/when we'll have both, this and ASI, it would be
cool to be able to configure the system in such a way that there is
only one active, to avoid paying the price of both! :-)

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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