Re: [Xen-devel] About block device mapping for guests

["M.A. Williamson" <maw48@xxxxxxxxx>]

> > It's technically possible to make the frontend behave more like a CD
> > drive, it's just that there's not a lot of demand for this, so it's
> > not implemented.
>
> Right, understood. Seems like this need would be better served by
> being able to give entire IDE/SCSI devices to some guests, so they can
> run all the IDE/SCSI commands they will ever need.
I guess you could do that but we'd need back / frontend drives that carried 
IDE / SCSI commands between guests and domain 0.
If you really want to give a whole device to a guest or make a CD writer 
work you could: * give the domain a whole PCI SCSI / IDE / USB controller 
and plug the device into that * export a USB drive to the domain with USB 
virtualisation
> I assume this specifically means that guest domains will not have the
> block devices created by devfs at all - but if one were to manually
> create the block devices with the correct major and minor, it would
> work.
Yes, that should work.

> Hmm. How will the devices be named under sysfs? If I just specify a
> major and minor device number, where does Xen dig up the name it
> should be registered under? And how does udev pick the right name for
> the actual device file?
First off, Xen doesn't know / care about the devices at all - it's the 
drivers in XenLinux that take care of that.
The guest operating system (e.g. XenLinux) will bind the major / minor to a 
name according to it's own policy. e.g. under Linux 0x801 is conventionally 
/dev/sda1 so unless you have a weird setup, that's what it'll appear as. I 
guess the files appear under SysFS similarly to how devices with the same 
major / minor would under vanilla Linux.
Udev knows the right name for the device file according to its own policy 
setup.
> Immensely. Now I know why none of my domains ever manage to find a
> root device ;-)
Ahh good ;-)

Cheers,
Mark


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