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

Kevin Wolf <kwolf(a)redhat.com&gt; writes: > 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. Correct. > 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. I can: $ qemu --enable-kvm -S -m 384 -vnc localhost:5500 -monitor stdio \ -drive if=none,id=cd,readonly=on,format=raw \ -device ide-cd,bus=ide.1,unit=0,drive=cd QEMU 1.1.50 monitor - type 'help' for more information (qemu) change cd r5.iso Could not open 'r5.iso' (qemu) q armbru@blackfin:~/work/images$ ls -l r5.iso -r--r--r--. 1 armbru armbru 2872639488 Mar 31 2011 r5.iso Looks like a QEMU bug to me.