x86/HVM: don't mark evtchn upcall vector as pending when vLAPIC is disabled

Linux'es relatively new use of HVMOP_set_evtchn_upcall_vector has
exposed a problem with the marking of the respective vector as
pending: For quite some time Linux has been checking whether any stale
ISR or IRR bits would still be set while preparing the LAPIC for use.
This check is now triggering on the upcall vector, as the registration,
at least for APs, happens before the LAPIC is actually enabled.

In software-disabled state an LAPIC would not accept any interrupt
requests and hence no IRR bit would newly become set while in this
state. As a result it is also wrong for us to mark the upcall vector as
having a pending request when the vLAPIC is in this state.

To compensate for the "enabled" check added to the assertion logic, add
logic to (conditionally) mark the upcall vector as having a request
pending at the time the LAPIC is being software-enabled by the guest.
Note however that, like for the pt_may_unmask_irq() we already have
there, long term we may need to find a different solution. This will be
especially relevant in case yet better LAPIC acceleration would
eliminate notifications of guest writes to this and other registers.

Fixes: 7b5b8ca7df ("x86/upcall: inject a spurious event after setting upcall vector")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
master commit: f5d0279839
master date: 2022-12-06 13:51:49 +0100

xen/arch/x86/hvm/irq.c

@@ -321,9 +321,10 @@ void hvm_assert_evtchn_irq(struct vcpu *v)
 
     if ( v->arch.hvm.evtchn_upcall_vector != 0 )
     {
-        uint8_t vector = v->arch.hvm.evtchn_upcall_vector;
+        struct vlapic *vlapic = vcpu_vlapic(v);
 
-        vlapic_set_irq(vcpu_vlapic(v), vector, 0);
+        if ( vlapic_enabled(vlapic) )
+           vlapic_set_irq(vlapic, v->arch.hvm.evtchn_upcall_vector, 0);
     }
     else if ( is_hvm_pv_evtchn_vcpu(v) )
         vcpu_kick(v);

xen/arch/x86/hvm/vlapic.c

@@ -828,6 +828,9 @@ void vlapic_reg_write(struct vcpu *v, unsigned int reg, uint32_t val)
         {
             vlapic->hw.disabled &= ~VLAPIC_SW_DISABLED;
             pt_may_unmask_irq(vlapic_domain(vlapic), &vlapic->pt);
+            if ( v->arch.hvm.evtchn_upcall_vector &&
+                 vcpu_info(v, evtchn_upcall_pending) )
+                vlapic_set_irq(vlapic, v->arch.hvm.evtchn_upcall_vector, 0);
         }
         break;