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Date:   Fri, 17 Apr 2020 15:08:25 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexander Graf <graf@amazon.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        vpillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, mingo@kernel.org,
        tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, fweisbec@gmail.com,
        keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.com>, Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>, aubrey.li@linux.intel.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>
Subject: Re: [RFC PATCH 00/13] Core scheduling v5
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On Fri, Apr 17, 2020 at 02:35:38PM +0200, Alexander Graf wrote:
> On 17.04.20 13:12, Peter Zijlstra wrote:

> If we first kick out the sibling HT for every #VMEXIT, performance will be
> abysmal, no?

I've been given to understand that people serious about virt try really
hard to avoid VMEXIT.


> > That doesn't completely solve things I think. Even if you run all
> > untrusted tasks as core exclusive, you still have a problem of them vs
> > interrupts on the other sibling.
> >
> > You need to somehow arrange all interrupts to the core happen on the
> > same sibling that runs your untrusted task, such that the VERW on
> > return-to-userspace works as intended.
> > 
> > I suppose you can try and play funny games with interrupt routing tied
> > to the force-idle state, but I'm dreading what that'll look like. Or
> > were you going to handle this from your irq_enter() thing too?
> 
> I'm not sure I follow. We have thread local interrupts (timers, IPIs) and
> device interrupts (network, block, etc).
> 
> Thread local ones shouldn't transfer too much knowledge, so I'd be inclined
> to say we can just ignore that attack vector.
> 
> Device interrupts we can easily route to HT0. If we now make "core
> exclusive" a synonym for "always run on HT0", we can guarantee that they
> always land on the same CPU, no?
> 
> Then you don't need to hook into any idle state tracking, because you always
> know which CPU the "safe" one to both schedule tasks and route interrupts to
> is.

That would come apart most mighty when someone does an explicit
sched_setaffinity() for !HT0.

While that might work for some relatively contained systems like
chromeos, it will not work in general I think.