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Re: Cache aliasing issues using 4K pages.

To: The Engineer <lper.home@gmail.com>
Subject: Re: Cache aliasing issues using 4K pages.
From: Thiemo Seufer <ths@networkno.de>
Date: Tue, 15 Jan 2008 01:58:14 +0000
Cc: linux-mips@linux-mips.org
In-reply-to: <1a18fe6d0801141225u2395ae6dj39d268014019b4a1@mail.gmail.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <1a18fe6d0801141225u2395ae6dj39d268014019b4a1@mail.gmail.com>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.5.17 (2007-12-11)
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

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