linux-kernel - Re: [RFC PATCH 0/2] x86: Fix missing core serialization on migration

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Date:   Tue, 14 Nov 2017 08:10:41 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Avi Kivity <avi@...lladb.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        maged michael <maged.michael@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrea Parri <parri.andrea@...il.com>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Greg Hackmann <ghackmann@...gle.com>,
        Will Deacon <will.deacon@....com>,
        David Sehr <sehr@...gle.com>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 0/2] x86: Fix missing core serialization on migration

On Tue, Nov 14, 2017 at 8:05 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Nov 14, 2017 at 03:17:12PM +0000, Mathieu Desnoyers wrote:
>> I've tried to create a small single-threaded self-modifying loop in
>> user-space to trigger a trace cache or speculative execution quirk,
>> but I have not succeeded yet. I suspect that I would need to know
>> more about the internals of the processor architecture to create the
>> right stalls that would allow speculative execution to move further
>> ahead, and trigger an incoherent execution flow. Ideas on how to
>> trigger this would be welcome.
>
> I thought the whole problem was per definition multi-threaded.
>
> Single-threaded stuff can't get out of sync with itself; you'll always
> observe your own stores.
>
> And ISTR the JIT scenario being something like the JIT overwriting
> previously executed but supposedly no longer used code. And in this
> scenario you'd want to guarantee all CPUs observe the new code before
> jumping into it.
>
> The current approach is using mprotect(), except that on a number of
> platforms the TLB invalidate from that is not guaranteed to be strong
> enough to sync for code changes.
>
> On x86 the mprotect() should work just fine, since we broadcast IPIs for
> the TLB invalidate and the IRET from those will get the things synced up
> again (if nothing else; very likely we'll have done a MOV-CR3 which will
> of course also have sufficient syncness on it).
>
> But PowerPC, s390, ARM et al that do TLB invalidates without interrupts
> and don't guarantee their TLB invalidate sync against execution units
> are left broken by this scheme.
>

On x86 single-thread, you can still get in trouble, I think.  Do a
store, get migrated, execute the stored code.  There's no actual
guarantee that the new CPU does a CR3 load due to laziness.