linux-kernel - Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync

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Date:   Fri, 2 Dec 2016 14:55:44 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Borislav Petkov <bp@...en8.de>, Borislav Petkov <bp@...nel.org>,
        Andy Lutomirski <luto@...nel.org>, Peter Anvin <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Matthew Whitehead <tedheadster@...il.com>,
        Henrique de Moraes Holschuh <hmh@....eng.br>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation

On Fri, Dec 2, 2016 at 1:10 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Dec 2, 2016 at 12:41 PM, Andy Lutomirski <luto@...capital.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.