xen/x86: Implement x86_apic_ops

Or rather just implement one different function as opposed
to the native one : the read function.

We synthesize the values.
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
[v1: Rebased on top of tip/x86/urgent]
[v2: Return 0xfd instead of 0xff in the default case]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/Makefile

@@ -20,5 +20,5 @@ obj-$(CONFIG_EVENT_TRACING) += trace.o
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_PARAVIRT_SPINLOCKS)+= spinlock.o
 obj-$(CONFIG_XEN_DEBUG_FS)	+= debugfs.o
-obj-$(CONFIG_XEN_DOM0)		+= vga.o
+obj-$(CONFIG_XEN_DOM0)		+= apic.o vga.o
 obj-$(CONFIG_SWIOTLB_XEN)	+= pci-swiotlb-xen.o

arch/x86/xen/apic.c

+#include <linux/init.h>
+#include <asm/x86_init.h>
+
+unsigned int xen_io_apic_read(unsigned apic, unsigned reg)
+{
+	if (reg == 0x1)
+		return 0x00170020;
+	else if (reg == 0x0)
+		return apic << 24;
+
+	return 0xfd;
+}
+
+void __init xen_init_apic(void)
+{
+	x86_io_apic_ops.read = xen_io_apic_read;
+}

arch/x86/xen/enlighten.c

@@ -1362,6 +1362,8 @@ asmlinkage void __init xen_start_kernel(void)
 		xen_start_info->console.domU.mfn = 0;
 		xen_start_info->console.domU.evtchn = 0;
 
+		xen_init_apic();
+
 		/* Make sure ACS will be enabled */
 		pci_request_acs();
 	}

arch/x86/xen/xen-ops.h

@@ -92,11 +92,15 @@ struct dom0_vga_console_info;
 
 #ifdef CONFIG_XEN_DOM0
 void __init xen_init_vga(const struct dom0_vga_console_info *, size_t size);
+void __init xen_init_apic(void);
 #else
 static inline void __init xen_init_vga(const struct dom0_vga_console_info *info,
 				       size_t size)
 {
 }
+static inline void __init xen_init_apic(void)
+{
+}
 #endif
 
 /* Declare an asm function, along with symbols needed to make it