Question about adding flags to mmap system call / NVIDIA amd64
marius at nuenneri.ch
Tue Apr 28 21:43:19 UTC 2009
On Tue, Apr 28, 2009 at 22:19, Julian Bangert <julidaoc at online.de> wrote:
> 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
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.
More information about the freebsd-hackers