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Prefetches in memcpy

To: Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org
Subject: Prefetches in memcpy
From: Carsten Langgaard <carstenl@mips.com>
Date: Tue, 05 Nov 2002 14:45:47 +0100
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
I have reported this before and it also gave a lot of responses, but
nothing has been done about, unfortunately :-(

The problem is the prefetches in the memcpy function in the kernel.
There is spread a number of PREF instructions in the memcpy function,
but there is no check if we are prefetching out-side the areas we are
copying to/from. This is extremely dangerous because we might prefetch
out-side the physical memory area, causing e.g. a bus error or something
even more nasty.

I recently found something even nastier, it could also hit a DMA buffer
region, and thereby break the PCI DMA flushing scheme.
For example if the kernel is doing a memcpy from an area that's next to
a DMA buffer area, we could end up in a situation where, we are
prefetching
data into the cache from a memory location that is used for DMA transfer
and owned by the device, but the DMA transfer has not yet completed.
We then end up in a situation, where the memory and cache is out of sync
and the cache is containing some old data.

So we definitely need to do something about the prefetches in the memcpy
function.
We can either get rid of all the prefetches or make sure we don't
prefetch out side the "memcpy" area.

/Carsten



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