On Mon, Jul 16, 2007 at 03:53:18PM +0400, Sergey Rogozhkin wrote:
> >Big loud bell began ringing. The RM7000 fetches and decodes multiple
> >instructions in one go. And just like the E9000 cores it does
> >throw an exception if it doesn't like one of the opcodes even if that
> >doesn't actually get executed. The kernel has a workaround for this
> >PMC-Sierra peculiarity (I call it a bug) but it's only being activated
> >for E9000 platforms.
> We have had a similar problems with shell on RM7000 based system. It
> seems, the reason listed above is only half of the problem, another is:
> linux works incorrectly with RM7000 caches hierarchy. One visible effect
> is errors in userspace on signal delivery trampolines.
> Lets imagine we deliver a signal to application: we write signal
> trampoline instructions to stack, writeback (and invalidate)
> corresponding dcache line, invalidate corresponding icache line. Thats
> all, and we think that we can safely execute the trampoline, but this is
> wrong on RM7000! Our trampoline is now in scache, and everything seems
> to be ok, but after some number of load/stores corresponding scache line
> can be moved to dcache, replaced in scache by another data and not
> written to memory (this is a feature of RM7000 caches, its dcache is not
> a subset of scache, you can find a possible scenario of similar (but not
> the same) cache line transference in RM7000 manual (7.1.5 Orphaned Cache
> Lines)). After that it is possible that on signal trampoline execution
> icache fetch old memory content instead of instruction written. If we
> want to execute instruction written by cpu, we must not only writeback
> corresponding dcache lines, but also writeback corresponding scache
> lines after it. The error is very sensitively to kernel/user code and
> data arrangement, it can be visible with one kernel configuration and
> irreproducible with another.
> The problem affects not only signal trampoline flush to memory, but most
> cases of icache invalidation in kernel.
Hmm... Makes sense. I guess I can cook up a patch based on that analysis.