[PATCH 4/4] qemu: validate: Prefer existing qemuCaps

[Michal Privoznik]

On 12/8/20 3:01 PM, Peter Krempa wrote:
> The validation callback always fetched a fresh copy of 'qemuCaps' to use
> for validation which is wrong in cases when the VM is already running,
> such as device hotplug. The newly-fetched qemuCaps may contain flags
> which weren't originally in use when starting the VM e.g. on a libvirtd
> upgrade.
> 
> Since the post-parse/validation machinery has a per-run 'parseOpaque'
> field filled with qemuCaps of the actual process we can reuse the caps
> in cases when we get them.
> 
> The code still fetches a fresh copy if parseOpaque doesn't have a
> per-run copy to preserve existing functionality.
> 
> Signed-off-by: Peter Krempa <pkrempa at redhat.com>
> ---
>   src/qemu/qemu_validate.c | 30 ++++++++++++++++++++----------
>   1 file changed, 20 insertions(+), 10 deletions(-)


So this will use the old qemuCaps even though a feature might already be 
available in qemu but libvirt didn't detect it. For instance the 
virtio-pmem I'm implementing - guests started today won't have the 
qemuCap for virtio-pmem because it's not implemented yet. But when they 
update libvirt they still won't get the feature because we're using old 
qemuCaps for validation and thus new memory model will be denied because 
of lacking capability.

I can see the value in when we have to chose between old and new style 
of doing things, but I worry it will kill new features.

Michal