Re: Cache aliasing issues using 4K pages.

[Thiemo Seufer <ths@networkno.de>]

Luc Perneel wrote:
> On Jan 15, 2008 2:58 AM, Thiemo Seufer <ths@networkno.de> wrote:
> 
> > The Engineer wrote:
> > > We are working with a 2.6.12 kernel on a dual-core mips architecture.
> > > In this dual-core system, one core is running the linux kernel and the
> > > other is used for some real-time handling (not directly controlled by
> > > Linux)
> > > We had different stability issues, which could be pinpointed to be
> > > related with cache aliasing problems.
> > > Cache aliasing happens when the same physical memory can be cached
> > > twice as it is accessed by two different virtual addresses.
> > > Indeed, for the index to select the correct cache line the virtual
> > > address is used. If some bits of the virtual page address are used in
> > > the cache index, aliasing can occur.
> > >
> > >
> > > As there is no hardware solution in the mips to recover from this
> > > (which would provide some cache coherency, even for one core), the
> > > only intrinsic safe solution is to enlarge the page size, so that
> > > cache indexing is only done by the offset address in the page (thus
> > > the physical part of the address).
> > > Another solution is to flush the cache if a page is being remapped to
> > > an aliased address (but in our case linux does not has control on the
> > > second core, which can cause issues with shared data between both
> > > cores).
> > > Currently the second solution is used in the kernel, but we found
> > > different issues with it (for instance: we had to merge more recent
> > > mips kernels, to get a reliable copy-on-write behaviour after
> > > forks...).
> > >
> > > Therefore some questions:
> > > - Are there still some known issues with cache aliasing in the MIPS
> > kernel?
> > > - Are there known issues when using 16KB pages (8KB pages seems not be
> > > possible due to tlb issues).
> >
> > With recent kernels and toolchains 16k pages work ok IME. With a 2.6.12
> > kernel however you'll have to backport a serious amount of bugfixes
> > before 16k pages can work. Upgrading the kernel is probably less work.
> >
> > Thiemo
> >
> Thanks for the info, any idea from which kernel this works ok?

I believe the linux-2.6.22-stable branch from www.linux-mips.org is the
best version to use. I don't recall if the earlier branches got all the
necessary fixes.


Thiemo