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

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Date:   Thu, 3 Sep 2020 16:30:14 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Dario Faggioli <dfaggioli@...e.com>,
        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>,
        Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <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,
        Steven Rostedt <rostedt@...dmis.org>,
        Patrick Bellasi <derkling@...gle.com>,
        benbjiang(蒋彪) <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 Thu, Sep 3, 2020 at 9:20 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Thu, Sep 03 2020 at 00:34, Joel Fernandes wrote:
> > On Wed, Sep 2, 2020 at 12:57 PM Dario Faggioli <dfaggioli@...e.com> wrote:
> >> 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! :-)
> >
> > Actually, no. Part of ASI will involve exactly what this patch does -
> > IPI-pausing siblings but ASI does so when they have no choice but to
> > switch away from the "limited kernel" mapping, into the full host
> > kernel mapping. I am not sure if they have yet implemented that part
> > but they do talk of it in [1] and in their pretty LPC slides.  It is
> > just that ASI tries to avoid that scenario of kicking all siblings out
> > of guest mode.  So, maybe this patch can be a stepping stone to ASI.
> > At least I got the entry hooks right, and the algorithm is efficient
> > IMO (useless IPIs are avoided).  ASI can then come in and avoid
> > sending IPIs even more by doing their limited-kernel-mapping things if
> > needed. So, it does not need to be this vs ASI, both may be needed.
>
> Right. There are different parts which are seperate:
>
> 1) Core scheduling as a best effort feature (performance for certain use
>    cases)
>
> 2) Enforced core scheduling (utilizes #1 basics)
>
> 3) ASI
>
> 4) Kick sibling out of guest/host and wait mechanics
>
> #1, #2, #3 can be used stand alone. #4 is a utility
>
> Then you get combos:
>
> A) #2 + #4:
>
>    core wide protection. i.e. what this series tries to achieve.  #3
>    triggers the kick at the low level VMEXIT or entry from user mode
>    boundary. The wait happens at the same level
>
> B) #3 + #4:
>
>    ASI plus kicking the sibling/wait mechanics independent of what's
>    scheduled. #3 triggers the kick at the ASI switch to full host
>    mapping boundary and the wait is probably the same as in #A
>
> C) #2 + #3 + #4:
>
>    The full concert, but trigger/wait wise the same as #B
>
> So we really want to make at least #4 an independent utility.

Agreed! Thanks for enlisting all the cases so well. I believe this
could be achieved by moving the calls to unsafe_enter() and
unsafe_exit() to when ASI decides it is time to enter the unsafe
kernel context.  I will keep it in mind when sending the next revision
as well.

thanks,

- Joel