I am working on an app where I want to give one or more user processes access to a largish range of physical address space (specifically, this is a Broadcom 1125 running a 32 bit kernel, and for now the region is accessible via KSEG0/1 (physical address < 512 MB)). mmap() on /dev/mem does this just fine, and setting (or not setting) O_SYNC on open seems to control caching. But I just realized a disadvantage to doing this in user space: the user process accesses have to be mapped (since a user process can't, I believe, use KSEG0 or KSEG1 addresses), so you have to go through the (64 entry) TLB, and if you had signficant non-locality of reference, you'd possibly risk thrashing the TLB (which doesn't happen in kernel space, since the region can be directly accessed). One approach would be to wire a TLB entry to handle the large region so you never get a TLB miss, but this might not work well for multi-process access, since (normally) you can't guarantee that the multiple processes doing mmap's will get the same virtual address. Is this correct? Is there some other clever approach I haven't thought of? Should I even be worrying about TLB usage? Thanks, sf
