vmx: VT-d posted-interrupt core logic handling

vmx: VT-d posted-interrupt core logic handling

This is the core logic handling for VT-d posted-interrupts. Basically it
deals with how and when to update posted-interrupts during the following
scenarios:
- vCPU is preempted
- vCPU is slept
- vCPU is blocked

When vCPU is preempted/slept, we update the posted-interrupts during
scheduling by introducing two new architecutral scheduler hooks:
vmx_pi_switch_from() and vmx_pi_switch_to(). When vCPU is blocked, we
introduce a new architectural hook: arch_vcpu_block() to update
posted-interrupts descriptor.

Besides that, before VM-entry, we will make sure the 'NV' filed is set
to 'posted_intr_vector' and the vCPU is not in any blocking lists, which
is needed when vCPU is running in non-root mode. The reason we do this check
is because we change the posted-interrupts descriptor in vcpu_block(),
however, we don't change it back in vcpu_unblock() or when vcpu_block()
directly returns due to event delivery (in fact, we don't need to do it
in the two places, that is why we do it before VM-Entry).

When we handle the lazy context switch for the following two scenarios:
- Preempted by a tasklet, which uses in an idle context.
- the prev vcpu is in offline and no new available vcpus in run queue.
We don't change the 'SN' bit in posted-interrupt descriptor, this
may incur spurious PI notification events, but since PI notification
event is only sent when 'ON' is clear, and once the PI notificatoin
is sent, ON is set by hardware, hence no more notification events
before 'ON' is clear. Besides that, spurious PI notification events are
going to happen from time to time in Xen hypervisor, such as, when
guests trap to Xen and PI notification event happens, there is
nothing Xen actually needs to do about it, the interrupts will be
delivered to guest atht the next time we do a VMENTRY.

Suggested-by: Yang Zhang <yang.z.zhang@intel.com>
Suggested-by: Dario Faggioli <dario.faggioli@citrix.com>
Suggested-by: George Dunlap <george.dunlap@citrix.com>
Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Feng Wu <feng.wu@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>