Re: Question about adding flags to mmap system call / NVIDIA amd64 driver implementation
- From: Marius Nünnerich <marius@xxxxxxxxxxx>
- Date: Tue, 28 Apr 2009 23:23:15 +0200
On Tue, Apr 28, 2009 at 22:19, Julian Bangert <julidaoc@xxxxxxxxx> 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.
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- From: Julian Bangert
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