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SubjectRe: Testing RAM from userspace / question about memmap= arguments
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)

--
If you want to reach me at my work email, use arjan@linux.intel.com
For development, discussion and tips for power savings,
visit http://www.lesswatts.org


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