Re: [libvirt] [PATCH] block: Set cdrom device read only flag

Am 02.08.2012 09:20, schrieb Kevin Shanahan: > On Thu, Aug 02, 2012 at 02:49:52PM +0930, Kevin Shanahan wrote: >> On Thu, Aug 02, 2012 at 11:46:13AM +0930, Kevin Shanahan wrote: >>> On Thu, Aug 02, 2012 at 11:02:42AM +0930, Kevin Shanahan wrote: >>>> Set the block driver read_only flag for cdrom devices so that >>>> qmp_change_blockdev does not attempt to open cdrom files in read-write >>>> mode when changing media. >>> >>> Hrm, this fixes my simple test case using the kvm monitor directly but >>> changing media via libvirt still has the same issue (fails for RO >>> files, succeeds for writable files). >>> >>> $ virsh attach-disk --type cdrom --mode readonly test1 >>> /srv/kvm/iso/ubuntu-12.04-server-amd64.iso hdc >>> error: Failed to attach disk >>> error: internal error unable to execute QEMU command 'change': >>> Could not open '/srv/kvm/iso/ubuntu-12.04-server-amd64.iso' >>> >>> I'll keep looking into it. >> >> In the libvirt case, it seems libvirt is failing to add media=cdrom to >> the commandline, so in this case qemu is defaulting to media=disk and >> my proposed fix has no effect. Diving into libvirt now to see why no >> media=disk is getting added... >> >> Common test case has this xml (generated by virt-install): >> >> <disk type='block' device='cdrom'> >> <driver name='qemu' type='raw'/> >> <target dev='hdc' bus='ide'/> >> <readonly/> >> <alias name='ide0-1-0'/> >> <address type='drive' controller='0' bus='1' target='0' unit='0'/> >> </disk> > > Ok, looks like libvirt is intentionally leaving media=cdrom off the > command line in the case that "-device ide-cd,..." is > supported. Presumably by specifying the device this way, qemu is > supposed to work out that the media type is cdrom automatically (but > it doesn't, it defaults to disk). > > Libvirt wants to use: > > qemu-kvm ... \ > -drive if=none,id=drive-ide0-1-0,readonly=on,format=raw \ > -device ide-cd,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 ... > > If I hack qemu/qemu_command.c::qemuBuildDriveStr() to ignore the check > for QEMU_CAPS_IDE_CD and always add media=cdrom, then (with my qemu as > well patch) qemu will open cdrom media files read-only. > > There's probably a neater way to just get qemu to set the media type > if "-device ide-cd,..." is used, but I haven't worked it out yet. > > Anyway, apologies for the rambling conversation with myself on your > lists. Hope this is helpful in some way. Thanks, that's some good information. However, I don't think you should start from media=cdrom. libvirt already does specify readonly=on and that is the property you're really interested in. Since commit 528f7663 (released with 0.13) the 'change' command should keep the read-only flag for all kinds of media.

Now I'm not sure where you lost the read-only flag. At least on git master it doesn't seem to reproduce for me.