[Xen-devel] [PATCH 4/4] x86/wakeup/sleep: Use pvops functions for changing GDT entries.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

From: "konrad@xxxxxxxxxx" <konrad@xxxxxxxxxx>

We check the TSS descriptor before we try to dereference it.
Also we document what the value '9' actually means using the
AMD64 Architecture Programmer's Manual Volume 2, pg 90:
"Hex value 9: Available 64-bit TSS" and pg 91:
"The available 32-bit TSS (09h), which is redefined as the
available 64-bit TSS."

Without this, on Xen, where the GDT is available as R/O (to
protect the hypervisor from the guest modifying it), we end up
with a pagetable fault.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 arch/x86/power/cpu.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
index 571176f..6d6e907 100644
--- a/arch/x86/power/cpu.c
+++ b/arch/x86/power/cpu.c
@@ -133,7 +133,10 @@ static void fix_processor_context(void)
 {
        int cpu = smp_processor_id();
        struct tss_struct *t = &per_cpu(init_tss, cpu);
-
+#ifdef CONFIG_X86_64
+       struct desc_struct *desc = get_cpu_gdt_table(cpu);
+       tss_desc tss;
+#endif
        set_tss_desc(cpu, t);   /*
                                 * This just modifies memory; should not be
                                 * necessary. But... This is necessary, because
@@ -142,7 +145,9 @@ static void fix_processor_context(void)
                                 */
 
 #ifdef CONFIG_X86_64
-       get_cpu_gdt_table(cpu)[GDT_ENTRY_TSS].type = 9;
+       memcpy(&tss, &desc[GDT_ENTRY_TSS], sizeof(tss_desc));
+       tss.type = 0x9; /* The available 64-bit TSS (see AMD vol 2, pg 91 */
+       write_gdt_entry(desc, GDT_ENTRY_TSS, &tss, DESC_TSS);
 
        syscall_init();                         /* This sets MSR_*STAR and 
related */
 #endif
-- 
1.8.1.4


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