[libvirt] [PATCH 08/10] qemu: Handle genid processing during startup/launch

[Daniel P. Berrangé]

On Wed, Apr 11, 2018 at 04:10:03PM -0400, John Ferlan wrote:
> Before we generate the command line for qemu, if the domain about to
> be launched desires to utilize the VM Generation ID functionality, then
> handle both the regenerating the GUID value for backup recovery (restore
> operation) and the startup after snapshot as well as checking that the
> genid value that's about to be placed on the command line doesn't
> duplicate some other already running domain.
> 
> Signed-off-by: John Ferlan <jferlan at redhat.com>
> ---
>  src/qemu/qemu_driver.c  |  22 +++++++---
>  src/qemu/qemu_process.c | 110 +++++++++++++++++++++++++++++++++++++++++++++++-
>  src/qemu/qemu_process.h |   1 +
>  3 files changed, 126 insertions(+), 7 deletions(-)


> +    /* Now let's make sure the genid this domain has is not duplicitous
> +     * with something else running. */
> +    if (virDomainObjListForEach(driver->domains, qemuProcessCheckGenID, vm) < 0)
> +        return -1;

Is this really required  ?   AFAIK, the gen ID merely has to be unique within
the scope of the installed guest disk image, not amongst all VMs that exist.
If it was the latter, the check we're doing is pretty weak because it only
considers one host, not VMs on every other host.   IMHO this check is not
desirable because it adds mutex contention against every guest, into the
start up path and other code paths too.

Regards,
Daniel
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