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Date: Tue, 6 Dec 2016 10:25:20 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Jan Beulich <JBeulich@suse.com>
Cc: Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [Xen-devel] [PATCH v3 4/4] x86/asm: Rewrite sync_core() to use
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Message-ID: <20161206092520.GX3092@twins.programming.kicks-ass.net>
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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.