Just a follow up:
I tried reading the address with lseek and read and it works correctly!
But when I use mmap it stops working. I tried mmaping a regular file to
make sure I was using mmap correctly. It seemed to work fine. I get
consistent non-mmaped/mmaped read results with the regular file, just
not with /dev/mem!
What could be wrong?
Marek
Marek Olszewski wrote:
Hello, I'm doing a little project that investigates the benefits of
using the new (seemingly only accessible from privileged code) x86
SSE3 monitor/mwait instructions for locks on hyperthreaded machines.
To get this to work, I need to allocate some contiguous memory in
kernel space that is memory mapped to some address range in a
userspace app. For now, I'm just trying to get this to work by
mmaping /dev/mem. I use virt_to_phys on a kmalloced address and
return it to my user app via a syscall. I then try to mmap /dev/mem
in my user app at an offset of the returned address. The memory
mapping seems to succeed, but when I check the contents of the address
(which I wrote to in kernel space), I don't get a match. Is there
some /dev/mem file location-to-physical memory address mapping that I
am not aware of? man mem says that they are the same.
I also tried returning a kernel address and mapping /dev/kmem. In
this case mmap doesn't even succeed.
Any ideas?
I'm running debian testing with linux 2.6.12.1
Also, I'm aware that mmap requires a page aligned offset. I take care
of this in all of my tests by mmaping memory at the first offset that
is page aligned preceding my desired address/offset.
Thanks in advance,
Marek
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