From: Matt Porter <mporter@kernel.crashing.org>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Kallol Biswas <Kallol_Biswas@pmc-sierra.com>, linuxppc-dev@ozlabs.org
Subject: Re: memory with __get_free_pages and disabling caching
Date: Fri, 24 Mar 2006 18:25:49 -0700 [thread overview]
Message-ID: <20060324182549.C20731@cox.net> (raw)
In-Reply-To: <1143248571.3710.33.camel@localhost.localdomain>; from benh@kernel.crashing.org on Sat, Mar 25, 2006 at 12:02:51PM +1100
On Sat, Mar 25, 2006 at 12:02:51PM +1100, Benjamin Herrenschmidt wrote:
>
> > Yes, that's how it works. After being allocated by the dma api
> > routines, the direct map is never accessed. Accessing the same
> > physical address via the cached direct map would cause serious
> > problems but you aren't allowed to touch address space like
> > that unless it's been allocated through a kernel allocator for
> > your use.
>
> That is still broken for at least 6xx CPUs ... they may well prefetch it
> and you die...
Right.
> For example, page A is a normal page allocated for kernel use, page B
> just a after A is used by the DMA allocator for uncacheable accesses
> (and is thus mapped twice). If something does a loop going through an
> array in page A, you have no guarantee that some smart prefetcher &
> speculative accesses will not bring bits of page B into the cache since
> it's mapped and cacheable...
We had a similar scenario on 4xx with the smart prefetching that is
used in copy_tofrom_user. It had to be modified not to prefetch
across pages for 4xx. Eugene first saw corruption due to adjacent
page prefetches into a dmaable page.
-Matt
next prev parent reply other threads:[~2006-03-25 1:25 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-03-24 23:44 memory with __get_free_pages and disabling caching Kallol Biswas
2006-03-25 0:20 ` Benjamin Herrenschmidt
2006-03-25 0:29 ` Matt Porter
2006-03-25 0:27 ` Matt Porter
2006-03-25 1:02 ` Benjamin Herrenschmidt
2006-03-25 1:25 ` Matt Porter [this message]
-- strict thread matches above, loose matches on Subject: below --
2006-03-24 19:13 Kallol Biswas
2006-03-24 19:29 ` Kumar Gala
2006-03-24 22:17 ` Paul Mackerras
2006-03-24 22:30 ` Benjamin Herrenschmidt
2006-03-24 2:15 Kallol Biswas
2006-03-24 3:05 ` Benjamin Herrenschmidt
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