linux-kernel - Re: Testing RAM from userspace / question about memmap= arguments

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Message-ID: <20071226021756.7ae9db08@laptopd505.fenrus.org>
Date:	Wed, 26 Dec 2007 02:17:56 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Matthew Bloch <matthew@...emark.co.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: Testing RAM from userspace / question about memmap= arguments

On Wed, 26 Dec 2007 00:09:57 +0100
Pavel Machek <pavel@....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)

-- 
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