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From: Filippo Sironi <filippo.sironi@gmail.com>
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Date: Sat, 3 Dec 2011 09:42:44 +0100
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Cc: Sironi Filippo <filippo.sironi@gmail.com>
Subject: mmap implementation for cdev
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Dear all,

I've a strange problem I cannot understand...
I implemented a piece of code within the FreeBSD 7.2 kernel that =
basically allocate a per-thread memory page to store a certain amount of =
information that must be read and written without crossing user to =
kernel and kernel to user boundaries.
To read and write the memory page I decided to allocate it in kernel =
space and then mmap'ing it to user space using a "virtual cdev" as an =
entry point for user space threads. The implementation of the mmap I =
came up with is really trivial, I basically do:

*paddr =3D vtophys(curthread->private_info);

Now for the problem.
I enter the mmap and private_info is the correct kernel space virtual =
address, each thread as its own address and the physical address - =
returned by vtophys - is different too. The problem is that when I come =
back to user space all the threads write in memory page mapped by the =
first thread that called the mmap leaving their memory pages untouched.

This problem bugs me but I cannot find a solution. I don't really =
understand the behavior.

Any ideas?
Thanks a lot,
Filippo