From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Matt Porter <mporter@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: Sat, 25 Mar 2006 12:02:51 +1100 [thread overview]
Message-ID: <1143248571.3710.33.camel@localhost.localdomain> (raw)
In-Reply-To: <20060324172733.A20731@cox.net>
> 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...
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...
Ben.
next prev parent reply other threads:[~2006-03-25 1:03 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 [this message]
2006-03-25 1:25 ` Matt Porter
-- 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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