Xen devel - Jan 2010 - Re: pvops dom0: no sound after boot; possibly causedby swiotlb

>On Tue, Jan 26, 2010 at 01:40:12AM +0100, Ronny.Hegewald@online.de wrote:
>> Software: xen 3.4.1, lastest xen/master (version 2.6.31.6), both 32bit
>> Hardware: Intel Core2Duo System
>>           4GB Ram
>>           Realtek ALC888 soundchip
>
>Motherboard?
MSI P35 Neo.
>
>What I think you are saying is that, when you perform these steps:
>
>1). Boot up machine, have swiotlb=16384 set (or just hand-coded
>swiotlb.c to have a default size of 32MB).
>
>2). Start a DomU with more than 66MB allocated to it.
>
>3). Load the sound modules in Dom0
>
>4). Play music/sound/etc in Dom0, the sound works fine.
Generally right except a little difference.

1.) if i let the swiotlb unchanged at 64 MB and start a DomU with 66 MB or more
Ram and do steps 3 and 4 the sound appears; with less RAM there is no sound
2.) if i lower the swiotlb to 32 MB and start a DomU with less then 66 MB Ram
and do steps 3 and 4, then the sound is fine.

>What I am curious is your E820 table. That is the first thing Xen
>prints. You can get that by doing ''xm dmesg''
>
>Also include your ''dmesg'' output for good measure. 
Attached.
>Lastly, try adding this line to your Xen line: dom0_mem=max:2GB and
>don''t do any of the above mentioned hacks. Just start the sound
module
>and see if it works.
I tried that too but used as parameter dom0_mem=2GB. But i will retry it anyway
because i tried many several things so i might have done something wrong in the
process.
>Back to the sound card. The sound card can (I think) only DMA up to 4GB,
>so when it tries to fetch data from above 4GB it ends up truncating the
>physical address down to 32-bit and fetches data from somewhere else. So
>you are probably listening to binary code being played :-)
In that case shouldnt i have that problem on bare metal too? And there is no
such problem under a 2.6.31 kernel with the forwardported dom0 patches from
gentoo either.
>Having a DomU start makes Xen shrink Dom0 by a certain size and the DMA
>window for the sound is moved "down" below the 4GB mark (that I
think is
>the problem you are encountering). This should NOT happen if the sound
>card driver is using DMA libraries to allocate that buffer (ie,
>dma_alloc_coherent, dma_alloc_free, etc). But it might be using
>''vmalloc_32'' which on virtualized environments is not
guaranteed to give
>a swatch of memory below 4GB. Here are the steps to investigate:
>look in the sound card drivers and see where and how it allocates the
buffer.
Thanks alot for all the explanations. Now i have a starting-point to do some
further investigations.

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