linux-kernel - Re: Whats the purpose of get_cycles

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 linux-cve-announce 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]

Message-ID: <20071030224234.GA19737@one.firstfloor.org>
Date:	Tue, 30 Oct 2007 23:42:34 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Vojtech Pavlik <vojtech@...e.cz>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Joerg Roedel <joerg.roedel@....com>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	benjamin.serebrin@....com
Subject: Re: Whats the purpose of get_cycles_sync()

On Tue, Oct 30, 2007 at 11:02:09PM +0100, Vojtech Pavlik wrote:
> > He can give details on the test.
> > 
> > I suspect the reason was because the CPU reordered the RDTSCs so that
> > a later RDTSC could return a value before an earlier one. This can
> > happen because gettimeofday() is so fast that a tight loop calling it can
> > fit more than one iteration into the CPU's reordering window.
> 
> The K8's still guarantee that subsequent RDTSCs return increasing
> values, even if the processor reorders them.

Ah didn't realize this

> 
> What could have been happening then was that the RDTSC instruction might
> have been reordered by the CPU out of the seqlock, causing trouble in
> the calculation.

Ok anyways it fixed that problem.  So it cannot be taken out.
> 
> Anyway, adding the CPUID didn't solve all the problems we've seen back
> then, and so far none of the approaches for using TSC without acquiring
> a spinlock on multi-socket AMD boxes worked 100% correctly.

The code is not used on multi-core anyways currently (without Jiri's
patch). It should just work correctly on single core.

-Andi
-
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/