Re: [libvirt] [PATCH v1 4/7] qemu: Allow regeneration of aliases

On Fri, Sep 22, 2017 at 10:18:02 +0200, Michal Privoznik wrote: > On 09/21/2017 07:18 PM, Daniel P. Berrange wrote: > > On Thu, Sep 21, 2017 at 06:05:19PM +0200, Peter Krempa wrote: > >> On Thu, Sep 21, 2017 at 16:47:08 +0200, Michal Privoznik wrote: > >>> In the near future the qemuAssignDeviceAliases() function is > >>> going to be called multiple times: once at the domain define > >>> time, then in domain prepare phase. To avoid regenerating the > >>> same aliases the second time we need to be able to tell the > >>> function which time it is being called. > >>> > >>> Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; > >>> --- > >>> src/qemu/qemu_alias.c | 113 ++++++++++++++++++++++++++++++++++++++++++++++-- > >>> src/qemu/qemu_alias.h | 4 +- > >>> src/qemu/qemu_driver.c | 2 +- > >>> src/qemu/qemu_process.c | 2 +- > >>> tests/qemuhotplugtest.c | 2 +- > >>> 5 files changed, 115 insertions(+), 8 deletions(-) > >>> > > >> So if you want to make aliases persistent, at least this will not work > >> properly with device coldplug. > > Ah, good point. > > >> > >> If you have two devices, detach the first one, then the second one moves > >> to index 0 but will still have alias ending with 1. Then if you cold-add > >> another device that will be put into index 1, and when starting the VM > >> it will get assigned the same alias as the one which has the old one. > >> > >> This applies to all devices where the alias depends on the ordering. > > > > Yep, we would need to maintain a hash table remembering all currently > > assigned aliases, and then increment the counter until we find a free > > one for that dev type. > > Alternatively, every time we want to assign an alias for a device we > traverse its siblings and see if it's taken. > > for (i = 0; ; i++) { > alias = "device$i"; > for (j = 0; j < def->ndevice; j++) { > if (STREQ(def->device[j]->info.alias, alias)) > continue; > break > } > if (j != def->ndevice) { > /* alias is free */ > device->alias = alias; > break; > } > } You can also generate them of a global sequence number. Qemu does not care that they are not consecutive. It might trigger some OCD based eye twitching, but it's way less yucky.