Re: virtual coherency issues with 4Kc ?

[Ralf Baechle <ralf@oss.sgi.com>]

On Fri, May 31, 2002 at 03:22:41PM -0700, Raymond Lo wrote:

> I'm evaluting the MIPS 4Kc core.   One thing I'm trying to find out is 
> how does linux deal with virtual aliasing in the cache for 4Kc.  
> 
> The cache of 4Kc is virtually-indexed and it has no hardware support to 
> suppress virtual aliasing.   The cachetlb.txt under linux/Documentation 
> indicates that two things need to be done in software to handle virtual 
> aliasing in D-cache.
> 
> The first is to handle virtual aliasing in user address spaces.  Shared 
> pages are mmaped at virtual addresses that are multiples of the cache 
> size.   That has already been taked care of in  include/asm-mips/shmparam.h.
> 
> The second is to handle virtual aliasing between kernel virtual address 
> space and user virtual address space by providing a number of functions 
> to flush the cache at various points in the kernel.   The old interface 
> is flush_page_to_ram.   The new ones are
>   copy_user_page,
>   clear_user_page,
>   flush_dcache_page.
> 
> I'm surprised to find out that flush_dcache_page is a macro defined to 
> be  do {} while (0) in linux/asm-mips/pgtable.h.  The source code I 
> looked is the web CVS on oss.sgi.com and 2.4.18.

We simply haven't converted to use the new interfaces yet.

> Apparently the necessary flushing hasn't been done from the mm and fs 
> code for any MIPS port.  I know this is not necessary for R4000 with 
> virtual coherency execptions.

It's still a good idea to not rely on the virtual coherency exception
mechanism which can result in a very significant overhead.

> P.S.   The link http://oss.sgi.com/mips/archive/ is stale.  

  Ralf