INTRODUCTION
============
Sometimes, it is useful to check what the driver is doing with
the PCI registers.

As it took me some time to get the right info from the Intenet,
and to write a few patches to enable debug, and to write them
to a file, I'm documenting here the procedure I used, as it
may be useful for the others.

One of the ways to see what a driver is doing with PCI registers
is to write several debug printk's. Another way is to enable PCI
device access at a VM machine, and catch the PCI emulation logs.

To use the second case, one of the ways is to modify the qemu-kvm
package. In my case, I wrote some patches for
qemu-kvm-0.12.1.2-2.113.el6.x86_64 (shipped with RHEL6), but they
probably apply on other versions of qemu.

Be careful: in order to get the PCI writes, the patches I'm using
intercepts all calls to PCI write/read to the mapped devices. This
is slow, and may cause machine hangs. If you're willing to use it,
you'll likely need to do on a separate machine, used just for
tests. On my test machine, they work fine for digital TV, but hangs
the machine with analog TV (probably because digital streams use
compressed traffic).

PATCHES FOR QEMU-KVM
====================

There are a few patches that are needed to allow getting a kvm log
from the PCI traffic.

The patches should be applied on this order:

kvm-allow-assigned-different-than-4K.patch
kvm-dev-assign-debug.patch
write_log_to_file.patch

1) kvm-allow-assigned-different-than-4K.patch

This patch came from qemu-kvm git tree. It will likely be available
on some stable version soon. It is needed for some devices.

2) kvm-dev-assign-debug.patch
This patch were written by me. It basically enables some debug
logs. It is licensed with GPLv2;

3) write_log_to_file.patch

This patch were written by me. It changes the debug logic to write
logs on a file, instead of stderr. The file is: /var/log/qemu_hw_pci.log.
You'll need to create it previously, change its ownership to match
qemu user, and fix selinux permissions (if you're using selinux).
In the case of RHEL6, I'm using:

-rw-r--r--. qemu qemu system_u:system_r:svirt_t:s0     qemu_hw_pci.log

IRQ CONFLICTS
=============

You need to be sure that IRQ's are not shared between the device you
want to sniff and any other device, otherwise, kvm won't allow you
to see the physical device. This is (currently) a kvm restriction.

You may check it  by looking at /proc/interrupts.

In my case, ehci_hcd and uhci_hcd shares interrrupts with devices I
plug on my PCI slots. So, I wrote a script to disconnect all USB buses
from my test machine. The script is:

	disconnect_usb.sh

MAPPING THE PHYSICAL DEVICE ON QEMU
===================================

You'll need to use an option for KVM to map the PCI device at the
machine. In my case, I'm using those lines:

-device pci-assign,host=37:09.0,id=hostdev0,configfd=22,bus=pci.0,addr=0x6
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5

The actual line depends on the PCI device that you're using. The above
example assumes a device at PCI address 37:09.0.

You may also use virt-manager to configure physical access to your PCI
device.

I hope this may be useful for you.
Mauro Carvalho Chehab <mchehab@kernel.org>