i386/xen: Implement SCHEDOP_poll and SCHEDOP_yield

They both do the same thing and just call sched_yield. This is enough to
stop the Linux guest panicking when running on a host kernel which doesn't
intercept SCHEDOP_poll and lets it reach userspace.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

target/i386/kvm/xen-emu.c

@@ -234,6 +234,19 @@ static bool kvm_xen_hcall_sched_op(struct kvm_xen_exit *exit, X86CPU *cpu,
         err = schedop_shutdown(cs, arg);
         break;
 
+    case SCHEDOP_poll:
+        /*
+         * Linux will panic if this doesn't work. Just yield; it's not
+         * worth overthinking it because with event channel handling
+         * in KVM, the kernel will intercept this and it will never
+         * reach QEMU anyway. The semantics of the hypercall explicltly
+         * permit spurious wakeups.
+         */
+    case SCHEDOP_yield:
+        sched_yield();
+        err = 0;
+        break;
+
     default:
         return false;
     }