On Tue, Apr 28, 2009 at 22:19, Julian Bangert <julid...@online.de> wrote: > Hello, > > I am currently trying to work a bit on the remaining "missing feature" that > NVIDIA requires ( http://wiki.freebsd.org/NvidiaFeatureRequests or a back > post in this ML) - the improved mmap system call. > For now, I am trying to extend the current system call and implementation > to add cache control ( the type of memory caching used) . This feature > inherently is very architecture specific- but it can lead to enormous > performance improvements for memmapped devices ( useful for drivers, etc). I > would do this at the user site by adding 3 flags to the mmap system call > (MEM_CACHE__ATTR1 to MEM_CACHE__ATTR3 ) which are a single octal digit > corresponding to the various caching options ( like Uncacheable,Write > Combining, etc... ) with the same numbers as the PAT_* macros from > i386/include/specialreg.h except that the value 0 ( PAT_UNCACHEABLE ) is > replaced with value 2 ( undefined), whereas value 0 ( all 3 flags cleared) > is assigned the meaning "feature not used, use default cache control". > For each cache behaviour there would of course also be a macro expanding to > the rigth combination of these flags for enhanced useability.
Hmm, I don't like that. What about using something like PAT_WC directly for the userland? Afaik a userland app that uses stuff like this is md anyway. > The mmap system call would, if any of these flags are set, decode them and > get a corresponding PAT_* value, perform the mapping and then call into the > pmap module to modify the cache attributes for every page. > > My first question is if there is a more elegant way of solving that - the 3 > flags would be architecture specific ( they could be used for other things > on other architectures though if need be ) and I do not know the policy on > architecture specific syscall flags, therefore I appreciate any input. > > The second question goes to all those great VM/pmap gurus out there: As far > as I understand, at the moment the pmap_change_attr can only cange the cache > flags for kernel pages. Is there a particular reason why this function might > not be adapted/extended to userspace mappings? If not, I would either add a > new function to iterate over all pages and set cache flags for a particular > region or add a new member (possibly just add the 3 flags again ? ) to the > md part of vm_page_t. Or one could just keep track and return errors as soon > as someone tries to map a memory region ( cache-customized mapping is > usually done to device memory ) already mapped with different cache > behaviour. Do you know how other OS handle this stuff? Maybe there is some inspiration there for a clean interface. I'm not sure if I remember correctly but there is something in my mind that we must take care that no virtual pages have different PAT settings for the same physical page. Maybe I read something like this in the AMD's documentation of PAT. Sorry I don't remember exactly but perhaps someone else can explain it better. _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
