x86/apic: fix disabling LVT0 in disconnect_bsp_APIC

The Intel SDM states:

"When an illegal vector value (0 to 15) is written to a LVT entry and
the delivery mode is Fixed (bits 8-11 equal 0), the APIC may signal an
illegal vector error, without regard to whether the mask bit is set or
whether an interrupt is actually seen on the input."

And that's exactly what's currently done in disconnect_bsp_APIC when
virt_wire_setup is true and LVT LINT0 is being masked. By writing only
APIC_LVT_MASKED Xen is actually setting the vector to 0 and the
delivery mode to Fixed (0), and hence it triggers an APIC error even
when the LVT entry is masked.

This would usually manifest when Xen is being shut down, as that's
where disconnect_bsp_APIC is called:

(XEN) APIC error on CPU0: 40(00)

Fix this by calling clear_local_APIC prior to setting the LVT LINT
registers which already clear LVT LINT0, and hence the troublesome
write can be avoided as the register is already cleared.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 782b48b7f7
master date: 2020-01-29 14:47:00 +0100

xen/arch/x86/apic.c

@@ -256,6 +256,8 @@ void disconnect_bsp_APIC(int virt_wire_setup)
         /* Go back to Virtual Wire compatibility mode */
         unsigned long value;
 
+        clear_local_APIC();
+
         /* For the spurious interrupt use vector F, and enable it */
         value = apic_read(APIC_SPIV);
         value &= ~APIC_VECTOR_MASK;
@@ -273,10 +275,6 @@ void disconnect_bsp_APIC(int virt_wire_setup)
             value = SET_APIC_DELIVERY_MODE(value, APIC_MODE_EXTINT);
             apic_write(APIC_LVT0, value);
         }
-        else {
-            /* Disable LVT0 */
-            apic_write(APIC_LVT0, APIC_LVT_MASKED);
-        }
 
         /* For LVT1 make it edge triggered, active high, nmi and enabled */
         value = apic_read(APIC_LVT1);