xen/arm: Introduce enable callback to enable a capabilities on each online CPU

Once Xen knows what features/workarounds present on the platform, it
might be necessary to configure each online CPU.

Introduce a new callback "enable" that will be called on each online CPU to
configure the "capability".

The code is based on Linux v4.14 (where cpufeature.c comes from), the
explanation of why using stop_machine_run is kept as we have similar
problem in the future.

Lastly introduce enable_errata_workaround that will be called once CPUs
have booted and before the hardware domain is created.

This is part of XSA-254.

Signed-of-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit 7500495155)

Conflicts:
	xen/include/asm-arm/cpufeature.h

(cherry picked from commit 2ec7ccbffc)

xen/arch/arm/cpuerrata.c

@@ -65,6 +65,12 @@ void check_local_cpu_errata(void)
 {
     update_cpu_capabilities(arm_errata, "enabled workaround for");
 }
+
+void __init enable_errata_workarounds(void)
+{
+    enable_cpu_capabilities(arm_errata);
+}
+
 /*
  * Local variables:
  * mode: C

xen/arch/arm/cpufeature.c

@@ -20,6 +20,7 @@
 #include <xen/types.h>
 #include <xen/init.h>
 #include <xen/smp.h>
+#include <xen/stop_machine.h>
 #include <asm/cpufeature.h>
 
 DECLARE_BITMAP(cpu_hwcaps, ARM_NCAPS);
@@ -40,6 +41,34 @@ void update_cpu_capabilities(const struct arm_cpu_capabilities *caps,
     }
 }
 
+/*
+ * Run through the enabled capabilities and enable() it on all active
+ * CPUs.
+ */
+void __init enable_cpu_capabilities(const struct arm_cpu_capabilities *caps)
+{
+    for ( ; caps->matches; caps++ )
+    {
+        if ( !cpus_have_cap(caps->capability) )
+            continue;
+
+        if ( caps->enable )
+        {
+            int ret;
+
+            /*
+             * Use stop_machine_run() as it schedules the work allowing
+             * us to modify PSTATE, instead of on_each_cpu() which uses
+             * an IPI, giving us a PSTATE that disappears when we
+             * return.
+             */
+            ret = stop_machine_run(caps->enable, (void *)caps, NR_CPUS);
+            /* stop_machine_run should never fail at this stage of the boot. */
+            BUG_ON(ret);
+        }
+    }
+}
+
 /*
  * Local variables:
  * mode: C

xen/arch/arm/setup.c

@@ -850,6 +850,7 @@ void __init start_xen(unsigned long boot_phys_offset,
      * stop_machine (tasklets initialized via an initcall).
      */
     apply_alternatives_all();
+    enable_errata_workarounds();
 
     /* Create initial domain 0. */
     /* The vGIC for DOM0 is exactly emulating the hardware GIC */

xen/include/asm-arm/cpuerrata.h

@@ -6,6 +6,7 @@
 #include <asm/alternative.h>
 
 void check_local_cpu_errata(void);
+void enable_errata_workarounds(void);
 
 #ifdef CONFIG_HAS_ALTERNATIVE
 

xen/include/asm-arm/cpufeature.h

@@ -72,6 +72,7 @@ struct arm_cpu_capabilities {
     const char *desc;
     u16 capability;
     bool_t (*matches)(const struct arm_cpu_capabilities *);
+    int (*enable)(void *); /* Called on every active CPUs */
     union {
         struct {    /* To be used for eratum handling only */
             u32 midr_model;
@@ -83,6 +84,8 @@ struct arm_cpu_capabilities {
 void update_cpu_capabilities(const struct arm_cpu_capabilities *caps,
                              const char *info);
 
+void enable_cpu_capabilities(const struct arm_cpu_capabilities *caps);
+
 #endif /* __ASSEMBLY__ */
 
 #endif