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

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Message-ID: <CA+55aFzeri_owd61Y+8EO9OjO5XJxZhHHrU=evN8moaiR+atUQ@mail.gmail.com>
Date:   Fri, 2 Dec 2016 13:10:31 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...capital.net>
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 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.

> (We *definitely* need to flush the store  buffer when migrating,

There is no such thing as flushing the store buffer.

But we do end up doing a memory barrier which gives you the required
semantics. That's not a problem. Those operations are fast. The
serializing instructions are not.

               Linus