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Re: [Libvir] RFC: format for mapping host devices to guest



On Thu, Jul 19, 2007 at 06:17:29AM -0400, Daniel Veillard wrote:
> On Thu, Jul 19, 2007 at 01:17:37AM +0100, Daniel P. Berrange wrote:
> > So to attach a device:
> > 
> >    <host bus='usb'>
> >       <source device="003.001"/>
> >    </host>
> > 
> > Or
> > 
> >    <host bus='usb'>
> >      <source vendor='0483' product='2016'>
> >    </host>
> > 
> > The first form is useful for live hotplug - where you know the exact device
> > instance. The second form is useful for permanent config - you don't know
> > what device id it'll have since it may be plugged into the host in any order.
> > 
> > When splitting the XML back out, QEMU would fill in the target info giving
> > us either:
> > 
> >    <host bus='usb'>
> >       <source dev='003.001'/>
> >       <target dev='001.005'/>
> >    </host>
> > 
> > Or
> > 
> >    <host bus='usb'>
> >       <source vendor='0483' product='2016'/>
> >       <target dev='001.005'/>
> >    </host>
> > 
> > This is suitable for giving to virDomainDetachDevice() - we use the target
> > dev for the QEMU monitor 'usb-detach' command. 
> 
>   Hum, that's the usual internal/external naming problem. Both have different
> context of use and life span. The vendor/product identifies the kind of device,
> it may not be sufficient to actually identify one of them (one could perfectly
> plug two usb mouse one for each side of keyboard), but it's the identifier
> which describe the functional requirement. And the other one is an internal
> name which may change over time.

Yep, I realize you could plug two of the same thing in - there's unfortunately
no way to distinguish. AFAIK qemu will just attach teh first device with matching
product & name, which is pretty much the best you can do. If you need reliable
naming then you'd have to do hotplug once the domain is running using the actual
device id instead of vendor+product.

> 
>   IMHO you should output both, then depending how you will reuse that XML
> fragment one or the other may be used, this could actually be one argument to
> split the device definition entry point and the device modification. For
> the definition you would use the vendor/product identifiers, while for example
> to simulate a disk change on an USB CDRom reader you would use device numbers.
> 
>     <host bus='usb'>
>        <device vendor='0471' product='0311' name='Philips PCVC740K ToUcam Pro'/>
>        <source dev='003.001'/>
>        <target dev='001.005'/>
>     </host>
> 
>   At the XML format you would require device or source to be present, possibly
> both and name would be an optional attribute (which need to be XML escaped).

The name doesn't belong here. I've got a separate email to write about a way
to enumerate devices on the host - currently virt-manager just calls HAL which
obviously doesn't work for remote management. So we need an API for enumerating
physical devices - the 'name' attribute for the devices is better placed in
there, rather the duplicating in the per-VM config.

> > Now PCI would be similar - no hotplug to worry about here though, and no
> > need (or availability) or target info. So the XML would simply be
> > 
> >    <host bus='pci'>
> >       <source dev='00:1f.3'/>
> >    </host>
> > 
> 
>   You may still have twice the same kind of card plugged on a PCI bus
> so just vendor/product doesn't sound sufficient to me. PCI device are
> more likely to be static, I would consider vendor/product to be user
> informations, but the code would always use the device if present.

That PCI dev ID is unique AFAIK - the 3 components uniquely identify the
PCI device encoding bus+slot or something like that - this is basically
the info from 'lspci' on Linux.

Dan.
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