Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

From: Michael S. Tsirkin
Date: Wed Jan 13 2016 - 12:00:27 EST

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.