Date: Tue, 28 Apr 2009 22:19:34 +0200 From: "Julian Bangert" <julidaoc@online.de> To: freebsd-hackers@freebsd.org Subject: Question about adding flags to mmap system call / NVIDIA amd64 driver implementation Message-ID: <op.us35euemeer2kn@server2go>
next in thread | raw e-mail | index | archive | help
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. 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. I thank you for your assistance & happy coding, --Julian Bangert
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?op.us35euemeer2kn>