Naive question about grant/map

[Volkmar Uhlig]

> -----Original Message-----
> From: Kevin Elphinstone 
> Sent: Wednesday, December 10, 2003 8:57 PM
> To: 'Jonathan S. Shapiro'
> Cc: 'L4 Hackers List'
> Subject: RE: Naive question about grant/map
> 
> Just a small, but important point relating to the statement below.
> 
> The L4 mapping database is NOT strictly a cache, but it can be used to
> implement cache-like behaviour which is usually what is done in most
> systems. Kernel mappings are a dynamic subset of things defined at
> user-level. However, in kernel mapping can be used to 
> indirectly define an address space or region of an address space 
> (if, for example, you what to avoid faults). I.e. it can equal 
> or be a superset of what is defined at user-level. However, doing 
> so requires carefully crafted and coordinated pager
behaviour/protocol. 
> 
> You can term it a (large) cache with mechanisms to control 
> its population, but it is not exactly a "cache" in the terms usual 
> sense of the word.

This also depends on your point of view.  Your statement that the kernel
mapping can be a superset of user-level pagers is wrong.  The user-level
pagers have full knowledge about the replacement policy (since it is
implemented in user land via map and unmap) and therefore can rely on
the fact that the kernel won't replace entries in the cache.  If it
would be a superset then the kernel would have to enter values itself.

- Volkmar