Re: CD-ROM size not updated when switching CD-ROM images.

[Alex Davis <alex14641@yahoo.com>]

--- On Wed, 10/20/10, David Ahern <dsahern@gmail.com> wrote:

> From: David Ahern <dsahern@gmail.com>
> Subject: Re: CD-ROM size not updated when switching CD-ROM images.
> To: "Alex Davis" <alex14641@yahoo.com>
> Cc: "Stefan Hajnoczi" <stefanha@gmail.com>, kvm@vger.kernel.org
> Date: Wednesday, October 20, 2010, 12:44 PM
> 
> 
> On 10/19/10 18:03, Alex Davis wrote:
> 
> >> You ejected the CD-ROM on the QEMU side, the guest
> doesn't
> >> necessarily
> >> know about the medium change.  What happens
> when you
> >> use "eject
> >> /dev/sr0" inside the guest instead?
> > I get a 'no medium found' error from the 'dd..'
> command. After
> > going into the the monitor, ejecting and reloading the
> disk, 
> > I still get the size of the first disk in the guest.
> 
> As I recall the old data is coming from the host page
> cache; it is not
> seeing the media change. Try dropping the cache in the host
> on a media
> change (echo 3 > /proc/sys/vm/drop_caches) or modify
> qemu to close and
> reopen the cdrom device.
> 
> David
I am able to reproduce this problem with just a plain kernel using the
ata_piix driver, an initrd that just starts a shell, and plain qemu.
When the disk is changed via the monitor, the guest invokes the error-
handling code on the next read of the CD-ROM, but the code path doesn't
trigger the 'update size' logic. I did an experiment where I placed the
line 'sdev->changed = 1' in the sr_open method in sr.c of the guest
kernel source, the size was updated, but I will look to see if there's
something in qemu that can be changed to make it work properly.

Of course any input from the qemu/kvm developers is appreciated.
 
I code, therefore I am

> 
> >>
> >> I don't know how CD-ROM media change works on
> real
> >> hardware, but that
> >> is the behavior that QEMU should be following.
> >>
> >> Stefan
> > 
> > 
> >       
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