RE: [Xen-users] Re: XEN sound emulation locks device exclusively

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Csillag, Kristof (GE Healthcare, consultant)
> Sent: 13 November 2006 07:42
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] Re: XEN sound emulation locks device exclusively
> 
> > As I think XEN has no "audio emulation", there's probably 
> no fix other than
> > using VMware that has an audio emulation.
> 
> OK, so I have the wrong idea about how XEN handles sound.
> (I found no documentation on this, so it was basically a guess.)
> 
> Here is what I know:
> 
> - In the notebook computer there is and Intel HDA sound card.
> - In the configuration file for my HVM guest, I have "soundhw='sb16'".
> - The WinXP running as HVM guest sees a Sound Blaster 16 
> sound device, and can use it.
> 
> Here is what I thought:
> 
> - Either the XEN hypervisor, or some user-space tool 
> (probably the later) emulates
>   the hardware interface of a SB16 for the HVM host, and
> - it relays the sound to the sound system of DOM0.
> 
> If this is not how it goes, then what?
> I thought that since there is no real SB16 in the host in question,
> the SB16 interface must have been emulated,
> but from what you say I seem to be mistaken.
> 
> Could someone please explain this, or give a link to the 
> appropriate documentation?

If it's a HVM guest, the sound emulation is done in QEMU-DM, the device
model for the HVM mode. 
In fact, qemu-dm is responsible for ALL hardware emulation with a few
exceptions (timer and interrupt controller functionality) that for
performance reasons are in Xen itself. 

So, yes, qemu-dm will have a SB16 type emulation layer that then
forwards the sound to be played to the real hardware. 

I would expect that this happens at a very low level, so it's "bare
samples" being sent to the hardware, which may mean that it needs to
lock the device exclusively to prevent other streams from interacting. 

--
Mats

> 
> Thank you:
> 
> 
>      Kristof Csillag
> 
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