pvops dom0: no sound after boot; possibly caused by swiotlb

[Ronny.Hegewald@online.de]

Software: xen 3.4.1, lastest xen/master (version 2.6.31.6), both 32bit
Hardware: Intel Core2Duo System
          4GB Ram
          Realtek ALC888 soundchip

Initial Symptoms:

When playing audio in dom0 there are just "knock" sounds.

After rmmod the kernel-module for the soundcard (snd-hda-intel) and oss modules (snd-seq-oss and snd-seq-pcm) and inserting them with modprobe again makes the sound work. Doing that only with snd-hda-intel doesnt help.

I compiled the sound-modules directly into the kernel but that didnt changed anything.

This problem doesnt appear with the gentoo-dom0 patches for kernel 2.6.31 so it looks like a pvops dom0 problem.

Strangely that problem doesnt appear on another system with the same xen-version and the exactly same kernel. Main-difference is that the other system is a 2-core AMD-system with 2 GB Ram and a different soundcard. 

But starting the domU with only 2 GB didnt made any difference.


Final findings:

It finally turns out that when the sound-modules are loaded after a pv-domU is started the sound in domU works fine from the beginning. 

As i suspected a problem in the memory-layout that got "fixed" by the start of a PV-domU i started the domU with different memory-sizes and found out that the sound works fine if the domU is started with at least 66 MB. Everything under that and there is no sound (even the knock-sound is not there)

The first thing i found that had could have to do with the 66 MB was the 64 MB swiotlb buffer. To check that this is really the problem i changed the code in arch/x86/xen/pci-swiotlb.c and lowered the allocated buffer to 32MB. After that change the sound worked from the beginning when the domU was started with less then 66 MB.


Further investigations:

>From here i dont know at the moment how to investigate that problem further myself.

Which logs should i post that could help to find the problem?

What further steps could/should i do to investigate that myself?