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.
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.
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