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From: Maxim Levitsky <maximlevitsky@gmail.com>
To: Arjan van de Ven <arjan@infradead.org>
Subject: Re: Testing RAM from userspace / question about memmap= arguments
Date: Wed, 26 Dec 2007 22:37:52 +0200
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Cc: Pavel Machek <pavel@ucw.cz>, Matthew Bloch <matthew@bytemark.co.uk>,
       linux-kernel@vger.kernel.org
References: <fk8umh$n7o$1@ger.gmane.org> <20071225230957.GB29030@elf.ucw.cz> <20071226021756.7ae9db08@laptopd505.fenrus.org>
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В сообщении от Wednesday 26 December 2007 12:17:56 Arjan van de Ven написал(а):
> On Wed, 26 Dec 2007 00:09:57 +0100
> Pavel Machek <pavel@ucw.cz> wrote:
> 
> > On Sat 2007-12-22 12:09:59, Arjan van de Ven wrote:
> > > On Tue, 18 Dec 2007 17:06:24 +0000
> 
> > > memtest86+ does various magic to basically bypass the caches (by
> > > disabling them ;-)... Doing that in a live kernel situation, and
> > > from userspace to boot...... that's... and issue.
> > 
> > Are you sure? I always assumed that memtest just used patterns bigger
> > than L1/L2 caches...
> 
> that's... not nearly usable or enough. Caches are relatively smart
> about things like use-once.... and they're huge. 12Mb today. You'd need
> patterns bigger than 100Mb to get even close to being reasonably
> confident that there's nothing left.
> 
> > ... and IIRC my celeron testing confirmed it, if
> > I disabled L2 cache in BIOS, memtest behave differently.
> > 
> > Anyway, if you can do iopl(), we may as well let you disable caches,
> > but you are right, that will need a kernel patch.
> 
> and a new syscall of some sorts I suspect; "flush all caches" is a ring
> 0 operation (and you probably need to do it in an ipi anyway on all
> cpus)
> 

I think that PAT support will help a lot.
How about opening/mmaping /dev/mem, and setting uncacheable attribute there.
Actually it is even possible today with MTRRs.

Regards,
	Maxim Levitsky
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