x86_64: add workaround for no %gs-based percpu

As a stopgap until Mike Travis's x86-64 gs-based percpu patches are
ready, provide workaround functions for x86_read/write_percpu for
Xen's use.

Specifically, this means that we can't really make use of vcpu
placement, because we can't use a single gs-based memory access to get
to vcpu fields.  So disable all that for now.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 8 insertions, 3 deletions

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index c97819829146..1b318e903bf6 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -39,6 +39,13 @@ static struct x8664_pda *__cpu_pda[NR_CPUS] __initdata;
 static struct x8664_pda *__cpu_pda[NR_CPUS] __read_mostly;
 #endif
 
+void __init x86_64_init_pda(void)
+{
+        _cpu_pda = __cpu_pda;
+        cpu_pda(0) = &_boot_cpu_pda;
+        pda_init(0);
+}
+
 static void __init zap_identity_mappings(void)
 {
         pgd_t *pgd = pgd_offset_k(0UL);
@@ -102,9 +109,7 @@ void __init x86_64_start_kernel(char * real_mode_data)
 
         early_printk("Kernel alive\n");
 
-        _cpu_pda = __cpu_pda;
-        cpu_pda(0) = &_boot_cpu_pda;
-        pda_init(0);
+        x86_64_init_pda();
 
         early_printk("Kernel really alive\n");