Re: [PATCH 11/12] hw/xen: automatically assign device index to block devices

[Kevin Wolf]

Am 23.10.2023 um 11:30 hat Igor Mammedov geschrieben:
> On Wed, 18 Oct 2023 09:32:47 +0100
> David Woodhouse <dwmw2@infradead.org> wrote:
> 
> > On Wed, 2023-10-18 at 09:32 +0200, Igor Mammedov wrote:
> > > On Mon, 16 Oct 2023 16:19:08 +0100
> > > David Woodhouse <dwmw2@infradead.org> wrote:
> > >   
> > > > From: David Woodhouse <dwmw@amazon.co.uk>
> > > >   
> > > 
> > > is this index a user (guest) visible?  
> > 
> > Yes. It defines what block device (e.g. /dev/xvda) the disk appears as
> > in the guest. In the common case, it literally encodes the Linux
> > major/minor numbers. So xvda (major 202) is 0xca00, xvdb is 0xca10 etc.
> 
> that makes 'index' an implicit ABI and a subject to versioning
> when the way it's assigned changes (i.e. one has to use versioned
> machine types to keep older versions working the they used to).
> 
> From what I remember it's discouraged to make QEMU invent
> various IDs that are part of ABI (guest or mgmt side).
> Instead it's preferred for mgmt side/user to provide that explicitly.
> 
> Basically you are trading off manageability/simplicity at QEMU
> level with CLI usability for human user.
> I don't care much as long as it is hidden within xen code base,
> but maybe libvirt does.

-drive is mostly a convenience option for human users anyway. Management
tools should use a combination of -blockdev and -device.

Kevin