No explanations just a few notes:
Install valgrind-mmt
git clone git://anongit.freedesktop.org/~z3ro/valgrind-mmt
Using valgrind-mmt
Then you need to find what the mapping of the FIFO is. Run X with the blob, check /proc/<pid>/maps for mappings of /dev/nvidia0, and note down the values.
Copy "Xorg" binary to e.g. Xorgmmt without the suid bit, then run valgrind:
valgrind --tool=mmt --offset=<mapping> Xorgmmt 2> /tmp/dump
Note by madman2003:
For PCI-E cards mappings move around a lot and you need to modify valgrind-mmt to match a range of offsets. In my case i had to watch the entire 0x10000000 range.