x86/apicv: fix RTC periodic timer and apicv issue

When Xen apicv is enabled, wall clock time is faster on Windows7-32
guest with high payload (with 2vCPU, captured from xentrace, in
high payload, the count of IPI interrupt increases rapidly between
these vCPUs).

If IPI intrrupt (vector 0xe1) and periodic timer interrupt (vector 0xd1)
are both pending (index of bit set in vIRR), unfortunately, the IPI
intrrupt is high priority than periodic timer interrupt. Xen updates
IPI interrupt bit set in vIRR to guest interrupt status (RVI) as a high
priority and apicv (Virtual-Interrupt Delivery) delivers IPI interrupt
within VMX non-root operation without a VM-Exit. Within VMX non-root
operation, if periodic timer interrupt index of bit is set in vIRR and
highest, the apicv delivers periodic timer interrupt within VMX non-root
operation as well.

But in current code, if Xen doesn't update periodic timer interrupt bit
set in vIRR to guest interrupt status (RVI) directly, Xen is not aware
of this case to decrease the count (pending_intr_nr) of pending periodic
timer interrupt, then Xen will deliver a periodic timer interrupt again.

And that we update periodic timer interrupt in every VM-entry, there is
a chance that already-injected instance (before EOI-induced exit happens)
will incur another pending IRR setting if there is a VM-exit happens
between virtual interrupt injection (vIRR->0, vISR->1) and EOI-induced
exit (vISR->0), since pt_intr_post hasn't been invoked yet, then the
guest receives more periodic timer interrupt.

So we set eoi_exit_bitmap for intack.vector - give a chance to post
periodic time interrupts when periodic time interrupts become the
highest one.

Signed-off-by: Quan Xu <xuquan8@huawei.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Chao Gao <chao.gao@intel.com>

xen/arch/x86/hvm/vmx/intr.c

@@ -312,12 +312,15 @@ void vmx_intr_assist(void)
         unsigned int i, n;
 
        /*
-        * Set eoi_exit_bitmap for periodic timer interrup to cause EOI-induced VM
-        * exit, then pending periodic time interrups have the chance to be injected
-        * for compensation
+        * intack.vector is the highest priority vector. So we set eoi_exit_bitmap
+        * for intack.vector - give a chance to post periodic time interrupts when
+        * periodic time interrupts become the highest one
         */
-        if (pt_vector != -1)
-            vmx_set_eoi_exit_bitmap(v, pt_vector);
+        if ( pt_vector != -1 )
+        {
+            ASSERT(intack.vector >= pt_vector);
+            vmx_set_eoi_exit_bitmap(v, intack.vector);
+        }
 
         /* we need update the RVI field */
         __vmread(GUEST_INTR_STATUS, &status);