[libvirt] [PATCH] qemu_command: Count the cpus correctly when the <vcpus> are absent

[Peter Krempa]

On Tue, Oct 04, 2016 at 17:22:37 +0530, Shivaprasad wrote:
> qemuBuildSmpCommandLine - is ending up generating "-smp 0," after
> first/subsequent restarts of the guest and we see "error: internal error: qemu
> reported thread id for inactive vcpu 'X'"
> 
> The PostParse logic which changes the hotplug = ABSENT to NO for each vcpus is
> not called with virDomainDefCopy(because of SKIP_POST_PARSE) after
> first/subsequent restarts of the guest. Skipping post-parse is fine for all
> devices as the changes done as part of previous post-parse are captured in
> virDomainDefFormat and effectively copied during virDomainDefCopy. With
> cpu-hotplug, the def->individualvcpus is what determines whether the <vcpus>
> are to be formatted or not in the xml. Since individualvcpus is not set, the
> copies will have hotpluggable as ABSENT.
> 
> The fix here is qemuBuildSmpCommandLine to check if the individualvcpus are
> set OR check the ABSENT flag. We dont want to set the individualvcpus in
> post-parse as that would disallow migration backwards to qemus not supporting
> hotplug. So, set check the ABSENT flag too when counting the vcpus instead.
> 
> Signed-off-by: Shivaprasad <sbhat at linux.vnet.ibm.com>
> ---
>  src/qemu/qemu_command.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Thanks for discovering this rather serious regression. Currently we've
decided to revert the patches that skip the post parse callbacks since
that's a more safe thing to do. After the release we'll revisit the post
parse callback change to do the proper thing, so this patch might not be
necessary then.

Peter