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>

4 files changed, 40 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");
 
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index a85f447b8d00..f3f11acf7856 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -971,6 +971,7 @@ void xen_setup_vcpu_info_placement(void)
 
         /* xen_vcpu_setup managed to place the vcpu_info within the
            percpu area for all cpus, so make use of it */
+#ifdef CONFIG_X86_32
         if (have_vcpu_info_placement) {
                 printk(KERN_INFO "Xen: using vcpu_info placement\n");
 
@@ -980,6 +981,7 @@ void xen_setup_vcpu_info_placement(void)
                 pv_irq_ops.irq_enable = xen_irq_enable_direct;
                 pv_mmu_ops.read_cr2 = xen_read_cr2_direct;
         }
+#endif
 }
 
 static unsigned xen_patch(u8 type, u16 clobbers, void *insnbuf,
@@ -1000,10 +1002,12 @@ static unsigned xen_patch(u8 type, u16 clobbers, void *insnbuf,
         goto patch_site
 
         switch (type) {
+#ifdef CONFIG_X86_32
                 SITE(pv_irq_ops, irq_enable);
                 SITE(pv_irq_ops, irq_disable);
                 SITE(pv_irq_ops, save_fl);
                 SITE(pv_irq_ops, restore_fl);
+#endif /* CONFIG_X86_32 */
 #undef SITE
 
         patch_site:
@@ -1323,6 +1327,7 @@ asmlinkage void __init xen_start_kernel(void)
 #ifdef CONFIG_X86_64
         /* Disable until direct per-cpu data access. */
         have_vcpu_info_placement = 0;
+        x86_64_init_pda();
 #endif
 
         xen_smp_init();
diff --git a/include/asm-x86/percpu.h b/include/asm-x86/percpu.h
index 912a3a17b9db..4e91ee1e37aa 100644
--- a/include/asm-x86/percpu.h
+++ b/include/asm-x86/percpu.h
@@ -22,6 +22,32 @@
 
 DECLARE_PER_CPU(struct x8664_pda, pda);
 
+/*
+ * These are supposed to be implemented as a single instruction which
+ * operates on the per-cpu data base segment.  x86-64 doesn't have
+ * that yet, so this is a fairly inefficient workaround for the
+ * meantime.  The single instruction is atomic with respect to
+ * preemption and interrupts, so we need to explicitly disable
+ * interrupts here to achieve the same effect.  However, because it
+ * can be used from within interrupt-disable/enable, we can't actually
+ * disable interrupts; disabling preemption is enough.
+ */
+#define x86_read_percpu(var)                                            \
+        ({                                                              \
+                typeof(per_cpu_var(var)) __tmp;                         \
+                preempt_disable();                                      \
+                __tmp = __get_cpu_var(var);                             \
+                preempt_enable();                                       \
+                __tmp;                                                  \
+        })
+
+#define x86_write_percpu(var, val)                                      \
+        do {                                                            \
+                preempt_disable();                                      \
+                __get_cpu_var(var) = (val);                             \
+                preempt_enable();                                       \
+        } while(0)
+
 #else /* CONFIG_X86_64 */
 
 #ifdef __ASSEMBLY__
diff --git a/include/asm-x86/setup.h b/include/asm-x86/setup.h
index 90ab2225e71b..659492624e74 100644
--- a/include/asm-x86/setup.h
+++ b/include/asm-x86/setup.h
@@ -76,6 +76,7 @@ extern unsigned long init_pg_tables_start;
 extern unsigned long init_pg_tables_end;
 
 #else
+void __init x86_64_init_pda(void);
 void __init x86_64_start_kernel(char *real_mode);
 void __init x86_64_start_reservations(char *real_mode_data);