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

[Peter Krempa]

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 at redhat.com>
> >>> ---
> >>>  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.


> This is roughly the same as your approach except the hash table is taken
> out. Which I find better because:
> 
> 1) the hash table would have to live under qemuDomainObjPrivatePtr (or
> under virDomainObjPtr) and I'd have to pass pointer to it all the way
> down to virDomainDeviceInfoClear()  -> huge change
> 
> 2) we would have two copies of aliases in memory.
> 
> IOW, my approach is more time complex but less memory complex.

Preferably as with disks or memory modules, you can base the alias on
something which is already guaranteed to be unique (IDE unit number,
DIMM slot number) which breaks the dependancy and gives you uniqueness
for free.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: not available
URL: <http://listman.redhat.com/archives/libvir-list/attachments/20170922/f1b3a97d/attachment-0001.sig>