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Re: Prefetches in memcpy

To: Ralf Baechle <ralf@uni-koblenz.de>
Subject: Re: Prefetches in memcpy
From: Carsten Langgaard <carstenl@mips.com>
Date: Wed, 04 Dec 2002 16:06:20 +0100
Cc: linux-mips@linux-mips.org, dom@mips.com, chris@mips.com, kevink@mips.com
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <3DC7CB8B.E2C1D4E5@mips.com> <20021105163806.A24996@bacchus.dhis.org>
Sender: linux-mips-bounce@linux-mips.org
I would like to open this thread again, since nothing has been done about it.
I think we should get rid of the prefetches until someone comes up with a
version that doesn't prefetch beyond the copy destination/source area.

/Carsten


Ralf Baechle wrote:

> On Tue, Nov 05, 2002 at 02:45:47PM +0100, Carsten Langgaard wrote:
>
> > 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.
>
> We could fix the prefetch into DMA buffer problem with an extra flush but
> that's going to be expensive, I rather think we should avoid prefetches.
> As Kevin explained KSEG1 is a loophole in the spec so we can't really say
> what the behaviour of memcpy will be in KSEG1.
>
> So I think the fix will have to be:
>
>  - Avoid prefetching beyond the end of the copy area in memcpy and memmove.
>  - Introduce a second variant of memcpy that never does prefetching.  This
>    one will be safe to use in KSEG1 / uncached XKPHYS also and will be used
>    for memcpy_fromio, memcpy_toio and friends.
>
>   Ralf

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