Re: [RFC PATCH 00/17] powerpc/e500: separate e500 from e500mc

[Moffett, Kyle D]

On Nov 10, 2011, at 23:40, Benjamin Herrenschmidt wrote:
> On Thu, 2011-11-10 at 18:38 -0600, Moffett, Kyle D wrote:
>>  (2) Make the ppc64_caches struct apply to ppc32 as well, and
>>      preinitialize it with a minimum value used by any platform being
>>      compiled in (for "dcbXX"/"icbXX" purposes).  This is safe because
>>      the pagesize is always a multiple of the cache block size and the
>>      kernel only uses dcbXX/icbXX on whole pages.  The only impact is a
>>      temporary small performance hit from flushing or zeroing the same
>>      block 8 times if too small.
> 
> Are you sure about dcbz ? Getting that wrong can be deadly ... I'd
> rather get rid of some fancy optims and use a soft value in some cases.
> That or we can compile multiple variants for the common case of some of
> the copy routines and use patching (alternate sections) to branch to the
> right one at runtime, at least for the common cases (32 and 128 for
> example for 440 and 476).

Well, all of the kernel loops that use dcbz are operating on whole pages,
and the PPC Book-E spec documents that the pagesize is an even multiple
of the cacheline size and the cachelines are always page-aligned.

So when you are clearing a whole page, there are only 2 things you can do
wrong with "dcbz":

  (1) Call "dcbz" with an address outside of the page you want to zero.

  (2) Omit calls "dcbz" to dcbz for some physical cachelines in the page.

Now, that's a totally different story from the userspace memset() calls
that caused the problem originally, because they were frequently given
memory much smaller than a page to clear, and if you didn't know exactly
how many bytes a "dcbz" was going to clear you couldn't use it at all.

But the kernel doesn't do that anywhere, it just uses it for page clears.

Cheers,
Kyle Moffett

--
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