how to operate io function in user mode?

[funggong]

hi,every body .
i confused that how an user mode driver using io ports in a microkernel env.
for example, a keyboard driver read a scancode using "in al,dx" instruction,but in ring 3 is not allow,i know tss io bitmap can do this,without change tss,is there another method can implement it. sorry for my english grammer and expression

[bluemoon]

I'm not doing microkernel but I can think of some possibilities:

1. kernel API - the kernel expose a hardware interface to driver, the actual IO is performed by kernel in ring 0.
2. TSS.io bitmap prevent direct IO, the kernel catch the #GP and emulate the io instruction
3. Driver request to grant access right and modify io bitmap.

I think 3 is more of the standard way, and there are nice tricks like mapping the bitmap via paging to avoid "modifying" TSS on every task switch.

[funggong]

I'm not doing microkernel but I can think of some possibilities:

1. kernel API - the kernel expose a hardware interface to driver, the actual IO is performed by kernel in ring 0.
2. TSS.io bitmap prevent direct IO, the kernel catch the #GP and emulate the io instruction
3. Driver request to grant access right and modify io bitmap.

I think 3 is more of the standard way, and there are nice tricks like mapping the bitmap via paging to avoid "modifying" TSS on every task switch.

thinks your reply.
but 3 will change tss every time scheduling thread which cost resource.

[funggong]

you gave me a great help,i will try it later

[Combuster]

but 3 will change tss every time scheduling thread which cost resource.

Code: Select all

  TSS (cpu1)
    IOPB ----------------+
  TSS (cpu2)             |
    IOPB ----------------+
  (...)                  |
-- page boundary --      |
  IO bitmap   <----------+

That way you can just reserve your zero-filled page for the IO bitmap space by default, and then replace the pages with actual content only for those drivers that need I/O ports. Changing address spaces will now give you a free IO bitmap update

[bluemoon]

but 3 will change tss every time scheduling thread which cost resource.

No, as I said there are tricks like page mapping, read Combuster's response for the concept.

[funggong]

understand now,that is said allocating a page for io bitmap which needed