Write to port 0cf8h fails with segfault

Question

I have an AMD processor of e2-2000 model. THis is family 0fh. According to family 0fh BKDG I have this code to read device and vendor ID:

ReadPCIConfiguration:
movq    $0x80000100, %rax
movq    $0x0cf8, %rdx
outl    %eax, %dx          # sigsegv caught here
movq    $0x0cfc, %rdx
inl     %dx, %eax
ret

As far as I know the algorithm to read/write PCI configuration is as follows:

• write target bus number, device number function number and offset or register number to configuration address port
• perform 1-, 2- or 4-byte r/w operation from/to configuration data port

Ports 0xcf8..0xcfb - configuration address port (doubleword)

Bits meaning:

• 31 - EnReg - enable the transaction (R/W)
• 24..31 - reserved (R/O)
• 16..23 - BusNum (R/W)
• 11..15 - DevNum (R/W)
• 8..10 - FuncNum (R/W)
• 2..7 - RegNum (R/W)
• 0..1 - Reserved (R/O)

So, I write to bus - 0, dev - 0, func - 1, reg - 00

Am I doing something wrong?

(I run the compiled and linked application from userspace, GNU/Debian "Wheezy" Linux 3.11.6)

Answer 1

Linux does not, by default, allow userland code to write to I/O ports. (Doing so can be quite dangerous from a security perspective.) If you’d like Linux to give your process access to the I/O ports, you have two options:

1. You can use the ioperm system call. However, ioperm has been deprecated for some time, and Josh Triplett recently pushed a patch that allows users to remove it from the kernel. Avoid ioperm if you want your code to continue working for the forseeable future.

2. You can read from and write to /dev/port. See mem(4). Your process will, obviously, need read and write permissions for /dev/mem; on Wheezy, that means it needs to run as root, unless you change the permissions on the device.