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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 2 Dec 2016 14:55:44 -0800
Message-ID: <CALCETrWu2PE2BWLzy28Fn3AEov5PJi=XOqVgf7ZSda1m7b35MQ@mail.gmail.com>
Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>, Borislav Petkov <bp@kernel.org>,
        Andy Lutomirski <luto@kernel.org>, Peter Anvin <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Matthew Whitehead <tedheadster@gmail.com>,
        Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>
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On Fri, Dec 2, 2016 at 1:10 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Dec 2, 2016 at 12:41 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> Because, if so, we should maybe serialize whenever we migrate a
>> process to a different CPU.
>
> The intel docs are bad on this issue.
>
> Technically what we do could fall under the "cross-modifying code"
> case, where one CPU does the write, and then we run it on another CPU.
>
> And no, we do *not* do a serializing instruction before returning to
> user space. Sure, we might do an iret (which is serializing), but we
> equally well might be doing a systret (which is not).
>
> Honestly, I think Intel should clean up their documentation.
>

I'm not sure I follow.  If a user program gets migrated, it might end
up doing cross-modification when it expects self-modification.  If
that trips the program up, is that a user bug or a kernel bug?

Admittedly, I'd be very surprised if this happened in practice.
Migration is *slow*, caches tend to get blown away, lots of code gets
executed, etc.  Presumably any prefetched / trace cached / decoded /
i-cached user code is long gone when we migrate.