ARM: spectre-v2: per-CPU vtables to work around big.Little systems

In big.Little systems, some CPUs require the Spectre workarounds in
paths such as the context switch, but other CPUs do not.  In order
to handle these differences, we need per-CPU vtables.

We are unable to use the kernel's per-CPU variables to support this
as per-CPU is not initialised at times when we need access to the
vtables, so we have to use an array indexed by logical CPU number.

We use an array-of-pointers to avoid having function pointers in
the kernel's read/write .data section.

Reviewed-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 23 insertions, 0 deletions

diff --git a/arch/arm/include/asm/proc-fns.h b/arch/arm/include/asm/proc-fns.h
index c259cc49c641..e1b6f280ab08 100644
--- a/arch/arm/include/asm/proc-fns.h
+++ b/arch/arm/include/asm/proc-fns.h
@@ -104,12 +104,35 @@ extern void cpu_do_resume(void *);
 #else
 
 extern struct processor processor;
+#if defined(CONFIG_BIG_LITTLE) && defined(CONFIG_HARDEN_BRANCH_PREDICTOR)
+#include <linux/smp.h>
+/*
+ * This can't be a per-cpu variable because we need to access it before
+ * per-cpu has been initialised.  We have a couple of functions that are
+ * called in a pre-emptible context, and so can't use smp_processor_id()
+ * there, hence PROC_TABLE().  We insist in init_proc_vtable() that the
+ * function pointers for these are identical across all CPUs.
+ */
+extern struct processor *cpu_vtable[];
+#define PROC_VTABLE(f)			cpu_vtable[smp_processor_id()]->f
+#define PROC_TABLE(f)			cpu_vtable[0]->f
+static inline void init_proc_vtable(const struct processor *p)
+{
+	unsigned int cpu = smp_processor_id();
+	*cpu_vtable[cpu] = *p;
+	WARN_ON_ONCE(cpu_vtable[cpu]->dcache_clean_area !=
+		     cpu_vtable[0]->dcache_clean_area);
+	WARN_ON_ONCE(cpu_vtable[cpu]->set_pte_ext !=
+		     cpu_vtable[0]->set_pte_ext);
+}
+#else
 #define PROC_VTABLE(f)			processor.f
 #define PROC_TABLE(f)			processor.f
 static inline void init_proc_vtable(const struct processor *p)
 {
 	processor = *p;
 }
+#endif
 
 #define cpu_proc_init			PROC_VTABLE(_proc_init)
 #define cpu_check_bugs			PROC_VTABLE(check_bugs)