diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index 5e3840084b..2443fd2cc5 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -21,6 +21,7 @@ obj-$(CONFIG_PV) += compat.o x86_64/compat.o
 obj-$(CONFIG_KEXEC) += crash.o
 obj-y += debug.o
 obj-y += delay.o
+obj-y += desc.o
 obj-bin-y += dmi_scan.init.o
 obj-y += domctl.o
 obj-y += domain.o
diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index ab2d52a79d..782deac0d4 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -2,7 +2,6 @@
 #include <xen/multiboot2.h>
 #include <public/xen.h>
 #include <asm/asm_defns.h>
-#include <asm/desc.h>
 #include <asm/fixmap.h>
 #include <asm/page.h>
 #include <asm/processor.h>
diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index 3909363ca3..f762dfea11 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -43,44 +43,11 @@ ENTRY(__high_start)
 multiboot_ptr:
         .long   0
 
-        .word   0
-GLOBAL(boot_gdtr)
-        .word   LAST_RESERVED_GDT_BYTE
-        .quad   boot_gdt - FIRST_RESERVED_GDT_BYTE
-
 GLOBAL(stack_start)
         .quad   cpu0_stack
 
         .section .data.page_aligned, "aw", @progbits
         .align PAGE_SIZE, 0
-GLOBAL(boot_gdt)
-        .quad 0x0000000000000000     /* unused */
-        .quad 0x00af9a000000ffff     /* 0xe008 ring 0 code, 64-bit mode   */
-        .quad 0x00cf92000000ffff     /* 0xe010 ring 0 data                */
-        .quad 0x0000000000000000     /* reserved                          */
-        .quad 0x00cffa000000ffff     /* 0xe023 ring 3 code, compatibility */
-        .quad 0x00cff2000000ffff     /* 0xe02b ring 3 data                */
-        .quad 0x00affa000000ffff     /* 0xe033 ring 3 code, 64-bit mode   */
-        .quad 0x00cf9a000000ffff     /* 0xe038 ring 0 code, compatibility */
-        .fill (PER_CPU_GDT_ENTRY - __HYPERVISOR_CS32 / 8 - 1), 8, 0
-        .quad 0x0000910000000000     /* per-CPU entry (limit == cpu)      */
-
-        .align PAGE_SIZE, 0
-/* NB. Even rings != 0 get access to the full 4Gb, as only the            */
-/*     (compatibility) machine->physical mapping table lives there.       */
-GLOBAL(boot_compat_gdt)
-        .quad 0x0000000000000000     /* unused */
-        .quad 0x00af9a000000ffff     /* 0xe008 ring 0 code, 64-bit mode   */
-        .quad 0x00cf92000000ffff     /* 0xe010 ring 0 data                */
-        .quad 0x00cfba000000ffff     /* 0xe019 ring 1 code, compatibility */
-        .quad 0x00cfb2000000ffff     /* 0xe021 ring 1 data                */
-        .quad 0x00cffa000000ffff     /* 0xe02b ring 3 code, compatibility */
-        .quad 0x00cff2000000ffff     /* 0xe033 ring 3 data                */
-        .quad 0x00cf9a000000ffff     /* 0xe038 ring 0 code, compatibility */
-        .fill (PER_CPU_GDT_ENTRY - __HYPERVISOR_CS32 / 8 - 1), 8, 0
-        .quad 0x0000910000000000     /* per-CPU entry (limit == cpu)      */
-        .align PAGE_SIZE, 0
-
 /*
  * Mapping of first 2 megabytes of memory. This is mapped with 4kB mappings
  * to avoid type conflicts with fixed-range MTRRs covering the lowest megabyte
diff --git a/xen/arch/x86/desc.c b/xen/arch/x86/desc.c
new file mode 100644
index 0000000000..c5dad90cdd
--- /dev/null
+++ b/xen/arch/x86/desc.c
@@ -0,0 +1,75 @@
+#include <xen/types.h>
+#include <xen/lib.h>
+#include <xen/percpu.h>
+#include <xen/mm.h>
+
+#include <asm/desc.h>
+
+/*
+ * Native and Compat GDTs used by Xen.
+ *
+ * The R1 and R3 descriptors are fixed in Xen's ABI for PV guests.  All other
+ * descriptors are in principle variable, with the following restrictions.
+ *
+ * All R0 descriptors must line up in both GDTs to allow for correct
+ * interrupt/exception handling.
+ *
+ * The SYSCALL/SYSRET GDT layout requires:
+ *  - R0 long mode code followed by R0 data.
+ *  - R3 compat code, followed by R3 data, followed by R3 long mode code.
+ *
+ * The SYSENTER GDT layout requirements are compatible with SYSCALL.  Xen does
+ * not use the SYSEXIT instruction, and does not provide a compatible GDT.
+ *
+ * These tables are used directly by CPU0, and used as the template for the
+ * GDTs of other CPUs.  Everything from the TSS onwards is unique per CPU.
+ */
+__section(".data.page_aligned") __aligned(PAGE_SIZE)
+seg_desc_t boot_gdt[PAGE_SIZE / sizeof(seg_desc_t)] =
+{
+    [ 1] = { 0x00af9a000000ffff }, /* 0xe008 - Ring 0 code, 64bit mode      */
+    [ 2] = { 0x00cf92000000ffff }, /* 0xe010 - Ring 0 data                  */
+                                   /* 0xe018 - reserved                     */
+    [ 4] = { 0x00cffa000000ffff }, /* 0xe023 - Ring 3 code, compatibility   */
+    [ 5] = { 0x00cff2000000ffff }, /* 0xe02b - Ring 3 data                  */
+    [ 6] = { 0x00affa000000ffff }, /* 0xe033 - Ring 3 code, 64-bit mode     */
+    [ 7] = { 0x00cf9a000000ffff }, /* 0xe038 - Ring 0 code, compatibility   */
+                                   /* 0xe040 - TSS                          */
+                                   /* 0xe050 - LDT                          */
+    [12] = { 0x0000910000000000 }, /* 0xe060 - per-CPU entry (limit == cpu) */
+};
+
+__section(".data.page_aligned") __aligned(PAGE_SIZE)
+seg_desc_t boot_compat_gdt[PAGE_SIZE / sizeof(seg_desc_t)] =
+{
+    [ 1] = { 0x00af9a000000ffff }, /* 0xe008 - Ring 0 code, 64-bit mode     */
+    [ 2] = { 0x00cf92000000ffff }, /* 0xe010 - Ring 0 data                  */
+    [ 3] = { 0x00cfba000000ffff }, /* 0xe019 - Ring 1 code, compatibility   */
+    [ 4] = { 0x00cfb2000000ffff }, /* 0xe021 - Ring 1 data                  */
+    [ 5] = { 0x00cffa000000ffff }, /* 0xe02b - Ring 3 code, compatibility   */
+    [ 6] = { 0x00cff2000000ffff }, /* 0xe033 - Ring 3 data                  */
+    [ 7] = { 0x00cf9a000000ffff }, /* 0xe038 - Ring 0 code, compatibility   */
+                                   /* 0xe040 - TSS                          */
+                                   /* 0xe050 - LDT                          */
+    [12] = { 0x0000910000000000 }, /* 0xe060 - per-CPU entry (limit == cpu) */
+};
+
+/*
+ * Used by each CPU as it starts up, to enter C with a suitable %cs.
+ * References boot_cpu_gdt_table for a short period, until the CPUs switch
+ * onto their per-CPU GDTs.
+ */
+struct desc_ptr boot_gdtr = {
+    .limit = LAST_RESERVED_GDT_BYTE,
+    .base = (unsigned long)(boot_gdt - FIRST_RESERVED_GDT_ENTRY),
+};
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/include/asm-x86/desc.h b/xen/include/asm-x86/desc.h
index 0be9348d29..0ebdcd05a9 100644
--- a/xen/include/asm-x86/desc.h
+++ b/xen/include/asm-x86/desc.h
@@ -103,10 +103,10 @@
 #define SYS_DESC_trap_gate    15
 
 typedef union {
+    uint64_t raw;
     struct {
         uint32_t a, b;
     };
-    uint64_t raw;
 } seg_desc_t;
 
 typedef union {
