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

From: Joel Fernandes <joel@joelfernandes.org>
To: Dario Faggioli <dfaggioli@suse.com>
Cc: "Thomas Gleixner" <tglx@linutronix.de>,
	"Julien Desfossez" <jdesfossez@digitalocean.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Vineeth Pillai" <viremana@linux.microsoft.com>,
	"Tim Chen" <tim.c.chen@linux.intel.com>,
	"Aaron Lu" <aaron.lwe@gmail.com>,
	"Aubrey Li" <aubrey.intel@gmail.com>,
	"Dhaval Giani" <dhaval.giani@oracle.com>,
	"Chris Hyser" <chris.hyser@oracle.com>,
	"Nishanth Aravamudan" <naravamudan@digitalocean.com>,
	"Ingo Molnar" <mingo@kernel.org>, "Paul Turner" <pjt@google.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Frederic Weisbecker" <fweisbec@gmail.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Greg Kerr" <kerrnel@google.com>, "Phil Auld" <pauld@redhat.com>,
	"Valentin Schneider" <valentin.schneider@arm.com>,
	"Mel Gorman" <mgorman@techsingularity.net>,
	"Pawan Gupta" <pawan.kumar.gupta@linux.intel.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	vineeth@bitbyteword.org, "Chen Yu" <yu.c.chen@intel.com>,
	"Christian Brauner" <christian.brauner@ubuntu.com>,
	"Agata Gruza" <agata.gruza@intel.com>,
	"Antonio Gomez Iglesias" <antonio.gomez.iglesias@intel.com>,
	graf@amazon.com, konrad.wilk@oracle.com,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"Patrick Bellasi" <derkling@google.com>,
	"benbjiang(蒋彪)" <benbjiang@tencent.com>,
	"Aubrey Li" <aubrey.li@linux.intel.com>,
	"Tim Chen" <tim.c.chen@intel.com>,
	"Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [RFC PATCH v7 17/23] kernel/entry: Add support for core-wide protection of kernel-mode
Date: Thu, 3 Sep 2020 16:25:05 -0400	[thread overview]
Message-ID: <CAEXW_YS-ozG2RnyUJ=OLi5cKJMDXRG+m5kY97d5rd-ubCfSD5A@mail.gmail.com> (raw)
In-Reply-To: <561110efffc3f6bf57894bf39861b39e1ec87845.camel@suse.com>

On Thu, Sep 3, 2020 at 9:43 AM Dario Faggioli <dfaggioli@suse.com> wrote:
>
> On Thu, 2020-09-03 at 00:34 -0400, Joel Fernandes wrote:
> > On Wed, Sep 2, 2020 at 12:57 PM Dario Faggioli <dfaggioli@suse.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.
> >
> Ah, sure! I mean, it of course depends on how things get mixed
> together, which we still don't know. :-)
>
> I know that ASI wants to do this very same thing some times. I just
> wanted to say that we need/want only one mechanism for doing that, in
> place at any given time.
>
> If that mechanism will be this one, and ASI then uses it (e.g., in a
> scenario where for whatever reason one wants ASI with the kicking but
> not Core Scheduling), then great, I agree, and everything does makes
> sense.

Agreed, with some minor modifications, it should be possible.

> > Why do you feel that ASI on its own offers some magical protection
> > that eliminates the need for this patch?
> >
> I don't. I was just trying to think of different use cases, how they
> mix and match, etc. But as you're suggesting (and as Thomas has shown
> very clearly in another reply to this email), the thing that makes the
> most sense is to have one mechanism for the kicking, used by anyone
> that needs it. :-)

Makes sense. Thanks,

 - Joel