[PATCH 12/18] qemu: capabilities: Add QEMU_CAPS_BLOCKDEV_HOSTDEV_SCSI

Peter Krempa pkrempa at redhat.com
Thu Jul 16 06:39:30 UTC 2020

On Thu, Jul 16, 2020 at 00:16:23 +0200, Ján Tomko wrote:
> On a Friday in 2020, Peter Krempa wrote:
> > We want to instantiate hostdevs via -blockdev too. Add a separate
> > capability for them for a clean transition. The new capability will be
> > enabled when QEMU_CAPS_BLOCKDEV is present once all code is prepared.
> > 
> What is the benefit here compared to using QEMU_CAPS_BLOCKDEV directly?

It encodes whether the particular VM instance was already started with
-blockdev used for hostdevs. The benefit ... or rather necessity is that
we need to handle cases when the VM was started with -drive differently
on hot-unplug of the hostdev.

I'd have to encode that bit of information somehow regardless of which
approach is used to actually store it.

> Seems more like a libvirt capability than a QEMU capability.

It is a capability of that particular instance of the VM the same way as
others. A VM started with pre-blockdev libvirt but modern qemu will not
use -blockdev for hotplug after libvirt upgrade and this is exactly te
same situation.

> Jano
> > Signed-off-by: Peter Krempa <pkrempa at redhat.com>
> > ---
> > src/qemu/qemu_capabilities.c | 1 +
> > src/qemu/qemu_capabilities.h | 1 +
> > 2 files changed, 2 insertions(+)

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