This could actually be very interesting. mmap() really does all of the
work. But notice that mmap() it is not passed any address information
(other than a "suggested" mapped, user-space address). It also received
an instance of the file descriptor of the driver.
The Linux file_operations structure is here:
https://elixir.bootlin.com/linux/latest/source/include/linux/fs.h#L1983
Notice that there is a mmap method in the file operations. So, mmap(),
it seems, calls the mmap method on the driver which performs the user
spacemapping of the device resource, much as you suggested in your
original email. The mmap() offset field specifies which device resource
to map.
That actually sounds very elegant to me and meets all of my concerns:
* The application cannot map any arbitrary kernel address to user
space so the implementation is secure.
* It uses the standard POSIX mmap() to accomplish the mapping.
On 3/28/2024 3:01 PM, Gregory Nutt wrote:
I found this document which describes the use cases for UIO in Linux:
https://www.kernel.org/doc/html/v4.13/driver-api/uio-howto.html . They
use mmap() to addess device memory: "|/dev/uioX| is used to access the
address space of the card. Just use |mmap()| to access registers or
RAM locations of your card."
There is a paragraph with a little more info:
https://www.kernel.org/doc/html/v4.13/driver-api/uio-howto.html#mmap-device-memory
My only concerns here are: (1) We use POSIX/Linux compatibility in
interface definitions and (2) We assure security of kernel resources.
I am not sure how mmap() knows which kernel mappings are secure and
which are insecure. But we would need to address that. If secure,
standard interfaces are used, then I am supportive (and you can ignore
many of my other comments which were just me fumbling to understand
the proposal).
On 3/28/2024 12:25 PM, Gregory Nutt wrote:
A more interesting task would be to port NxLib to run on top of the
existing NuttX NX Windows System:
* https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=139629474
* https://cwiki.apache.org/confluence/display/NUTTX/NX+Graphics+Subsystem
That would not be particularly easier, however. The NX Window System
is purely a windowing system and does not provide any significant
graphics support. X11, by comparison, is a complete graphics system
and user libraries are really only needed to simplify the interfaces
and to extend the graphics capabilities.
You would have to come up with some way to provide the graphics
capability in Nano-X not included in NX Windows.
In the NX Windows world, graphics was intended to be provided by
NxWidgets. That, however, is a work that was never really completed
to production quality. There are also several window managers that
use NX Windows and NxWidgets: NX and Twm4Nx. The latter is a TWM
inspired window manager (but a unique C++ development).
