On Apr 28, 2009, at 3:19 PM, Julian Bangert 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.
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.
Have you looked at mem(4) yet?
Several architectures allow attributes to be associated with
ranges of
physical memory. These attributes can be manipulated via
ioctl() calls
performed on /dev/mem. Declarations and data types are to be
found in
<sys/memrange.h>.
The specific attributes, and number of programmable ranges may
vary
between architectures. The full set of supported attributes is:
MDF_UNCACHEABLE
The region is not cached.
MDF_WRITECOMBINE
Writes to the region may be combined or performed out of
order.
MDF_WRITETHROUGH
Writes to the region are committed synchronously.
MDF_WRITEBACK
Writes to the region are committed asynchronously.
MDF_WRITEPROTECT
The region cannot be written to.
This requires knowledge of the physical addresses, but I believe
that's probably already necessary for what it sounds like you're
trying to accomplish.
Back in the FreeBSD-3.0 days, I was writing a custom driver for an AGP
graphics controller, and setting the MTRR flags for the exposed buffer
was a definite improvement (200-1200% faster in most cases).
-- Kevin
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