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

[Nuutti Kotivuori <naked@xxxxxx>]

Mark Williamson wrote:
> Yes you can choose an arbitrary IDE or SCSI drive's major / minor
> pair and export as that.  There's no difference to the backend
> driver, the frontend in the guest uses different blocksize for fake
> IDE vs fake SCSI, I think.

Right. Uh. I'll take a peek at the code to get the exact answer.

> 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.

> Guest domains can't use devfs because the frontend driver doesn't
> support it.  The developers feeling is that since devfs is
> deprecated, it's probably not worth the pain of adding support for.

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.

I can understand that it might not be worth the effort - but larger
warnings somewhere that 'devfs will not work, plain dev or udev will
work' might be in order.

> Yes, all Xen virtual devices should appear in SysFS in the guest,
> which udev will use to do the right thing - this should all work
> fine.

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?

> In the guest, sysfs will show block devices with whatever major /
> minor was specified.  It won't tell you what the drive is mapped to
> in dom0.
>
> State regarding which block devices in dom0 are mapped to what in
> the guests is maintained inside Xend.

Right.

> HTH,

Immensely. Now I know why none of my domains ever manage to find a
root device ;-)

-- Naked


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