spapr/xive: Allocate vCPU IPIs from the vCPU contexts

spapr/xive: Allocate vCPU IPIs from the vCPU contexts

When QEMU switches to the XIVE interrupt mode, it creates all the
guest interrupts at the level of the KVM device. These interrupts are
backed by real HW interrupts from the IPI interrupt pool of the XIVE
controller.

Currently, this is done from the QEMU main thread, which results in
allocating all interrupts from the chip on which QEMU is running. IPIs
are not distributed across the system and the load is not well
balanced across the interrupt controllers.

Change the vCPU IPI allocation to run from the vCPU context. The
associated XIVE IPI interrupt will be allocated on the chip on which
the vCPU is running and improve distribution of the IPIs in the system.
When the vCPUs are pinned, this will make the IPI local to the chip of
the vCPU. It will reduce rerouting between interrupt controllers and
gives better performance.

Device interrupts are still treated the same. To improve placement, we
would need some information on the chip owning the virtual source or
the HW source in case of a passthrough device but this reuires
changes in PAPR.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20200820134547.2355743-5-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>