lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Wed, 11 Nov 2009 10:35:24 +0100
From:	Willy Tarreau <w@....eu>
To:	Pavel Machek <pavel@....cz>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matteo Croce <technoboy85@...il.com>,
	Sven-Haegar Koch <haegar@...net.de>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: i686 quirk for AMD Geode

On Wed, Nov 11, 2009 at 09:03:13AM +0100, Pavel Machek wrote:
> 
> > (...)
> > > > CMOV/NOPL are rarely used, thus have no reason to cause a massive
> > > > performance drop, but are frequent enough (at least cmov) for almost
> > > 
> > > *One* CMOV in the inner loop will make your performance go down 20x.
> > 
> > yes, just like with emulated FPU or trapped unaligned accesses. It's
> > just like flying fishes. They exist but they aren't the most common
> > ones. If people encounter these cases on a specific program, then
> > they just have to recompile it if it is a problem. At least they
> > don't rebuild the whole distro. And once again, I've been using
> > cmpxchg/bswap emulation for years on my i386 without feeling any
> > need for a rebuild, and CMOV emulation for years now on my mini-itx 
> 
> And did you set cpu family to 6 for your 386?

No, not at all !

> That's the part I was objecting most. Yes, you can emulate, but
> emulation is very bad for performance... so don't lie about cpu
> family.

That's the point I agree with you, I don't like this setting either.
I don't want to incite applications to use features that are emulated.
However I think that allowing applications run when they don't/can't
perform the check is useful.

> (Imagine application that has NOPL in inner loop, for performance
> reasons. You want to use version _without_ the NOPL on processors that
> lack it.)

Exactly. Same for MMX, 3DNOW, SSE* and such.

> So... I don't like instruction emulation, but can live with it. But
> don't lie about supported instructions in /proc as original patch did.

100% agreed !

Regards,
Willy

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists