KVM: x86/xen: Add event channel interrupt vector upcall

KVM: x86/xen: Add event channel interrupt vector upcall

It turns out that we can't handle event channels *entirely* in userspace
by delivering them as ExtINT, because KVM is a bit picky about when it
accepts ExtINT interrupts from a legacy PIC. The in-kernel local APIC
has to have LVT0 configured in APIC_MODE_EXTINT and unmasked, which
isn't necessarily the case for Xen guests especially on secondary CPUs.

To cope with this, add kvm_xen_get_interrupt() which checks the
evtchn_pending_upcall field in the Xen vcpu_info, and delivers the Xen
upcall vector (configured by KVM_XEN_ATTR_TYPE_UPCALL_VECTOR) if it's
set regardless of LAPIC LVT0 configuration. This gives us the minimum
support we need for completely userspace-based implementation of event
channels.

This does mean that vcpu_enter_guest() needs to check for the
evtchn_pending_upcall flag being set, because it can't rely on someone
having set KVM_REQ_EVENT unless we were to add some way for userspace to
do so manually.

But actually, I don't quite see how that works reliably for interrupts
injected with KVM_INTERRUPT either. In kvm_vcpu_ioctl_interrupt() the
KVM_REQ_EVENT request is set once, but that'll get cleared the first time
through vcpu_enter_guest(). So if the first exit is for something *else*
without interrupts being enabled yet, won't the KVM_REQ_EVENT request
have been consumed already and just be lost?

I wonder if my addition of '|| kvm_xen_has_interrupt(vcpu)' should
actually be '|| kvm_has_injectable_intr(vcpu)' to fix that pre-existing
bug?

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>