linux-kernel - Re: [Xen-devel] [PATCH v3 4/4] x86/asm: Rewrite sync

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Date:   Tue, 6 Dec 2016 10:25:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Brian Gerst <brgerst@...il.com>,
        Matthew Whitehead <tedheadster@...il.com>,
        Henrique de Moraes Holschuh <hmh@....eng.br>,
        x86@...nel.org, xen-devel <Xen-devel@...ts.xen.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <JGross@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH v3 4/4] x86/asm: Rewrite sync_core() to use
 IRET-to-self

On Tue, Dec 06, 2016 at 01:46:37AM -0700, Jan Beulich wrote:
> > +	asm volatile (
> > +		"pushfl\n\t"
> > +		"pushl %%cs\n\t"
> > +		"pushl $1f\n\t"
> > +		"iret\n\t"
> > +		"1:"
> > +		: "+r" (__sp) : : "cc", "memory");
> 
> I don't thing EFLAGS (i.e. "cc") gets modified anywhere here. And
> the memory clobber would perhaps better be pulled out into an
> explicit barrier() invocation (making it more obvious what it's needed
> for)?

EVerything that implies a memory barrier (and I think serializing
instructions do that) also imply a compiler barrier.

Not doing the memory clobber gets you inconsistency wrt everything else.