hw/xen: select kernel mode for per-vCPU event channel upcall vector

A guest which has configured the per-vCPU upcall vector may set the
HVM_PARAM_CALLBACK_IRQ param to fairly much anything other than zero.

For example, Linux v6.0+ after commit b1c3497e604 ("x86/xen: Add support
for HVMOP_set_evtchn_upcall_vector") will just do this after setting the
vector:

       /* Trick toolstack to think we are enlightened. */
       if (!cpu)
               rc = xen_set_callback_via(1);

That's explicitly setting the delivery to GSI#1, but it's supposed to be
overridden by the per-vCPU vector setting. This mostly works in Qemu
*except* for the logic to enable the in-kernel handling of event channels,
which falsely determines that the kernel cannot accelerate GSI delivery
in this case.

Add a kvm_xen_has_vcpu_callback_vector() to report whether vCPU#0 has
the vector set, and use that in xen_evtchn_set_callback_param() to
enable the kernel acceleration features even when the param *appears*
to be set to target a GSI.

Preserve the Xen behaviour that when HVM_PARAM_CALLBACK_IRQ is set to
*zero* the event channel delivery is disabled completely. (Which is
what that bizarre guest behaviour is working round in the first place.)

Cc: qemu-stable@nongnu.org
Fixes: 91cce756179 ("hw/xen: Add xen_evtchn device for event channel emulation")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/hw/i386/kvm/xen_evtchn.c b/hw/i386/kvm/xen_evtchn.c
index a731738411..3d6f4b4a0a 100644
--- a/hw/i386/kvm/xen_evtchn.c
+++ b/hw/i386/kvm/xen_evtchn.c
@@ -490,6 +490,12 @@ int xen_evtchn_set_callback_param(uint64_t param)
         break;
     }
 
+    /* If the guest has set a per-vCPU callback vector, prefer that. */
+    if (gsi && kvm_xen_has_vcpu_callback_vector()) {
+        in_kernel = kvm_xen_has_cap(EVTCHN_SEND);
+        gsi = 0;
+    }
+
     if (!ret) {
         /* If vector delivery was turned *off* then tell the kernel */
         if ((s->callback_param >> CALLBACK_VIA_TYPE_SHIFT) ==