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, 1 insertions, 0 deletions

diff --git a/include/sysemu/kvm_xen.h b/include/sysemu/kvm_xen.h
index 595abfb..961c702 100644
--- a/include/sysemu/kvm_xen.h
+++ b/include/sysemu/kvm_xen.h
@@ -22,6 +22,7 @@
 int kvm_xen_soft_reset(void);
 uint32_t kvm_xen_get_caps(void);
 void *kvm_xen_get_vcpu_info_hva(uint32_t vcpu_id);
+bool kvm_xen_has_vcpu_callback_vector(void);
 void kvm_xen_inject_vcpu_callback_vector(uint32_t vcpu_id, int type);
 void kvm_xen_set_callback_asserted(void);
 int kvm_xen_set_vcpu_virq(uint32_t vcpu_id, uint16_t virq, uint16_t port);