On Wed, 3 Jun 1998 10:17:22 -0600 
 mhitch@lightning.oscs.montana.edu (Michael L. Hitch) wrote:

 >   I was going to ask if there would ever be a userspace address.  I don't
 > know of any case where it might be - as far as I know, any I/O to userspace
 > is done through kernel virtual addresses.  The one case that will currently
 > cause problems on the mips is if the address is on the kernel stack.  The
 > kernel stack is still being mapped at UADDR, and the mapping may change
 > between process context switches.

There isn't currently anything that does DMA directly to a user space
address, but the bus_dma specification _allows_ it (and in my paper for
USENIX, I provide an example of how it might be used).

 >   The only problem I see with Jason's change is the same problem I have with
 > some of the cache flushing in pmap:  it flushes all the entries within the
 > virtually-indexed range.  This could be flushing entries containing
 > different physical addresses/tags, which is not necessary.  I don't see
 > any easy way around that though.

Yah, sigh.  But, what to do about flushing for user space addresses for
non-curproc?  Grovel the pmap's software page tables and look into the
cache registers?

Jason R. Thorpe                                       thorpej@nas.nasa.gov
NASA Ames Research Center                            Home: +1 408 866 1912
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