linux-kernel - Re: [PATCH 3/4] x86,asm: Re-work smp_store

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Message-ID: <20160113185644-mutt-send-email-mst@redhat.com>
Date:	Wed, 13 Jan 2016 19:00:13 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Davidlohr Bueso <dbueso@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

On Wed, Jan 13, 2016 at 05:53:20PM +0100, Borislav Petkov wrote:
> On Wed, Jan 13, 2016 at 06:42:48PM +0200, Michael S. Tsirkin wrote:
> > Oh, I think this means we need a "cc" clobber.
> 
> Btw, does your microbenchmark do it too?

Yes - I fixed it now, but it did not affect the result.
We'd need some code where gcc carries flags around though.

> Because, the "cc" clobber should cause additional handling of flags,
> depending on the context. It won't matter if the context doesn't need
> rFLAGS handling in the benchmark but if we start using LOCK; ADD in the
> kernel, I can imagine some places where mb() is used and rFLAGS are
> live, causing gcc to either reorder code or stash them away...

It will reorder code but not necessarily for the worse :)
Best I can do, I will add cc clobber to kernel and see whether
binary size grows.

> -- 
> Regards/Gruss,
>     Boris.
> 
> ECO tip #101: Trim your mails when you reply.