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 -- _ _ ____ ___ Carsten Langgaard Mailto:carstenl@mips.com |\ /|||___)(___ MIPS Denmark Direct: +45 4486 5527 | \/ ||| ____) Lautrupvang 4B Switch: +45 4486 5555 TECHNOLOGIES 2750 Ballerup Fax...: +45 4486 5556 Denmark http://www.mips.com
