Re: [Xen-devel] [RFC] Virtual disk configuration, PV vs. emulated, backward compatibility etc

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 29 Jul 2010, Ian Jackson wrote:
> Jeremy Fitzhardinge writes ("Re: [Xen-devel] [RFC] Virtual disk 
> configuration, PV vs. emulated, backward compatibility etc"):
> >   On 07/28/2010 09:05 AM, Ian Jackson wrote:
> > > For Linux HVM guests using PV-on-HVM drivers, users are recommended to
> > > supply as few hd* devices as possible and use pure xvd* devices for
> > > the rest.  Modern PV-on-HVM drivers will map the hd* devices to
> > > /dev/xvdHDa etc.
> > 
> > I think we've decided to make blkfront register pv versions of emulated 
> > devices as hdX/sdX rather than using xvdHD.  We don't do this in pv domains.
> 
> Stealing the major number from the ide and scsi drivers, or just the
> name ?
> 

Both

> What if the domain has real sd* devices too ?  (pvscsi, pvusb + usb
> mass storage, passthrough, ...)
> 

Clashes are theoretically possible but very hard to produce in practice.
We are "stealing" device names only for emulated IDE and SCSI disks, and
emulated SCSI disks don't even work at the moment. So you would need to
passthrough an IDE controller whose disks are configured as hd* (most
distros use sd* for IDE disks).
I think we are doing exactly what the user asked us to: setting up an
hdX device; in these very unlikely scenarios the user knows what he is
doing and can change the configuration.

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