whpx: x86: kick out of HLT manually when using the kernel-irqchip

Otherwise, interrupts processed through the cancel vCPU and inject path will not cause the vCPU to go out of its halt state.

Signed-off-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Link: https://lore.kernel.org/r/20260226181930.53170-3-mohamed@unpredictable.fr
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/whpx/whpx-all.c

@@ -1468,6 +1468,16 @@ static void whpx_vcpu_post_run(CPUState *cpu)
         !vcpu->exit_ctx.VpContext.ExecutionState.InterruptShadow;
 }
 
+static void whpx_vcpu_kick_out_of_hlt(CPUState *cpu) 
+{
+    WHV_REGISTER_VALUE reg;
+    whpx_get_reg(cpu, WHvRegisterInternalActivityState, &reg);
+    if (reg.InternalActivity.HaltSuspend) {
+        reg.InternalActivity.HaltSuspend = 0;
+        whpx_set_reg(cpu, WHvRegisterInternalActivityState, reg);
+    }
+}
+
 static void whpx_vcpu_process_async_events(CPUState *cpu)
 {
     X86CPU *x86_cpu = X86_CPU(cpu);
@@ -1775,6 +1785,25 @@ int whpx_vcpu_run(CPUState *cpu)
                 cpu->exception_index = EXCP_INTERRUPT;
                 ret = 1;
             }
+            /* 
+             * When the Hyper-V APIC is enabled, to get out of HLT we
+             * either have to request an interrupt or manually get it away
+             * from HLT.
+             *
+             * We also manually do inject some interrupts via WHvRegisterPendingEvent
+             * instead of WHVRequestInterrupt, which does not reset the HLT state.
+             *
+             * However, even with this done, if the guest does an HLT without
+             * interrupts enabled (which the test_sti_inhibit KVM unit test does)
+             * then the guest will stay in HLT forever.
+             *
+             * Keep it this way for now, with perhaps adding a heartbeat later
+             * so that we get the CPU time savings from having Hyper-V handle HLT
+             * instead of going away from it as soon as possible.
+             */
+            if (whpx_irqchip_in_kernel()) {
+                whpx_vcpu_kick_out_of_hlt(cpu);
+            }
             break;
         case WHvRunVpExitReasonX64MsrAccess: {
             WHV_REGISTER_VALUE reg_values[3] = {0};