Xen imposes special requirements on the GDT. Rather than using a
global variable for the pgprot, just use an explicit special case
for Xen -- this makes it clearer what's going on. It also debloats
64-bit kernels very slightly.
Cc: Boris Ostrovsky <boris.ostrov...@oracle.com>
Cc: Juergen Gross <jgr...@suse.com>
Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
arch/x86/include/asm/desc.h | 1 -
arch/x86/kernel/cpu/common.c | 28 +++++++++++++++++-----------
arch/x86/xen/enlighten.c | 3 ---
3 files changed, 17 insertions(+), 15 deletions(-)
diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 17cb46e8a184..d0a21b12dd58 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -39,7 +39,6 @@ extern struct desc_ptr idt_descr;
extern gate_desc idt_table[];
extern const struct desc_ptr debug_idt_descr;
extern gate_desc debug_idt_table[];
-extern pgprot_t pg_fixmap_gdt_flags;
struct gdt_page {
struct desc_struct gdt[GDT_ENTRIES];
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index f6e20e2dbfa5..8ee32119144d 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -448,21 +448,27 @@ void load_percpu_segment(int cpu)
load_stack_canary_segment();
}
-/*
- * On 64-bit the GDT remapping is read-only.
- * A global is used for Xen to change the default when required.
- */
+/* Setup the fixmap mapping only once per-processor */
+static inline void setup_fixmap_gdt(int cpu)
+{
#ifdef CONFIG_X86_64
-pgprot_t pg_fixmap_gdt_flags = PAGE_KERNEL_RO;
+ /* On 64-bit systems, we use a read-only fixmap GDT. */
+ pgprot_t prot = PAGE_KERNEL_RO;
#else
-pgprot_t pg_fixmap_gdt_flags = PAGE_KERNEL;
+ /*
+ * On native 32-bit systems, the GDT cannot be read-only because
+ * our double fault handler uses a task gate, and entering through
+ * a task gate needs to change an available TSS to busy. If the GDT
+ * is read-only, that will triple fault.
+ *
+ * On Xen PV, the GDT must be read-only because the hypervisor requires
+ * it.
+ */
+ pgprot_t prot = boot_cpu_has(X86_FEATURE_XENPV) ?
+ PAGE_KERNEL_RO : PAGE_KERNEL;
#endif
-/* Setup the fixmap mapping only once per-processor */
-static inline void setup_fixmap_gdt(int cpu)
-{
- __set_fixmap(get_cpu_gdt_ro_index(cpu), get_cpu_gdt_paddr(cpu),
- pg_fixmap_gdt_flags);
+ __set_fixmap(get_cpu_gdt_ro_index(cpu), get_cpu_gdt_paddr(cpu), prot);
}
/* Load the original GDT from the per-cpu structure */
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 08faa61de5f7..4951fcf95143 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1545,9 +1545,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
*/
xen_initial_gdt = &per_cpu(gdt_page, 0);
- /* GDT can only be remapped RO */
- pg_fixmap_gdt_flags = PAGE_KERNEL_RO;
-
xen_smp_init();
#ifdef CONFIG_ACPI_NUMA
--
2.9.3
