MIME-Version: 1.0
In-Reply-To: <CALCETrXo1zLXcRX-tF-MQ5qa7sLrKoAYdn5ox-F-kp-3JhPaXg@mail.gmail.com>
References: <cover.1480638597.git.luto@kernel.org> <cover.1480536936.git.luto@kernel.org>
 <0a21157c2233ba7d0781bbf07866b3f2d7e7c25d.1480638597.git.luto@kernel.org>
 <CA+55aFw3P++jGbRhkrM3MyqwZZ3mxmxQTC1BfYS2rhw6P3LKNA@mail.gmail.com>
 <CALCETrXCLLW9pd=h7dZ0u2oBSiRBJOBkxvEq2iz=Z8aTVtrZ=w@mail.gmail.com>
 <20161202180343.gehqor7lgtmzwqq3@pd.tnic> <CA+55aFyHptQ3=gjXA_LGGkvOgsRB9FBXvCT56fG4UqcFV4wFsA@mail.gmail.com>
 <20161202185008.tdziqrzi4a3axord@pd.tnic> <CA+55aFxuhzE0woFyRjZ8=Ji1EPR1+MohrbDm=2AiQH50dsptjg@mail.gmail.com>
 <20161202192050.l5l3rcwems6hptub@pd.tnic> <CA+55aFydg4RDv8a-Wpf=TyzPN59kONcUdCkk1KcSNMW_2Uix_g@mail.gmail.com>
 <CALCETrVtEg256EPMbp1j8RkbaMJNtNge6-h0EoZ3HmRo6DZCLQ@mail.gmail.com>
 <CA+55aFwwa_A3RfodonHVDz+2xZ0+x+=MRPCVDqTkaG96SV5ZoQ@mail.gmail.com> <CALCETrXo1zLXcRX-tF-MQ5qa7sLrKoAYdn5ox-F-kp-3JhPaXg@mail.gmail.com>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 2 Dec 2016 13:10:31 -0800
Message-ID: <CA+55aFzeri_owd61Y+8EO9OjO5XJxZhHHrU=evN8moaiR+atUQ@mail.gmail.com>
Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation
To: Andy Lutomirski <luto@amacapital.net>
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>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 921
Lines: 25

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.

> (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