[RFC PATCH 5/6] powerpc/dexcr: Add sysctl entry for SBHE system override

[Benjamin Gray]

The DEXCR Speculative Branch Hint Enable (SBHE) aspect controls whether
the hints provided by BO field of Branch instructions are obeyed during
speculative execution.

SBHE behaviour per ISA 3.1B:

0:	The hints provided by BO field of Branch instructions may be
	ignored during speculative execution

1:	The hints provided by BO field of Branch instructions are obeyed
	during speculative execution

Add a sysctl entry to allow changing this aspect globally in the system
at runtime:

	/proc/sys/kernel/speculative_branch_hint_enable

Three values are supported:

-1:	Disable DEXCR SBHE sysctl override
 0:	Override and set DEXCR[SBHE] aspect to 0
 1:	Override and set DEXCR[SBHE] aspect to 1

Internally, introduces a mechanism to apply arbitrary system wide
overrides on top of the prctl() config.

Signed-off-by: Benjamin Gray <bgray at linux.ibm.com>
---
 arch/powerpc/include/asm/processor.h |  8 +--
 arch/powerpc/kernel/dexcr.c          | 85 ++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index a9d83621dfad..e7b732efb968 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -461,14 +461,14 @@ int exit_vmx_usercopy(void);
 int enter_vmx_ops(void);
 void *exit_vmx_ops(void *dest);
 
-static inline unsigned long get_thread_dexcr(struct thread_struct const *thread)
-{
 #ifdef CONFIG_PPC_BOOK3S_64
-	return thread->dexcr_enabled;
+unsigned long get_thread_dexcr(struct thread_struct const *thread);
 #else
+static inline unsigned long get_thread_dexcr(struct thread_struct const *thread)
+{
 	return 0;
-#endif
 }
+#endif
 
 #endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
diff --git a/arch/powerpc/kernel/dexcr.c b/arch/powerpc/kernel/dexcr.c
index db663ce7b3ce..e790f76787db 100644
--- a/arch/powerpc/kernel/dexcr.c
+++ b/arch/powerpc/kernel/dexcr.c
@@ -1,7 +1,9 @@
 #include <linux/capability.h>
+#include <linux/cpu.h>
 #include <linux/init.h>
 #include <linux/prctl.h>
 #include <linux/sched.h>
+#include <linux/sysctl.h>
 
 #include <asm/cpu_has_feature.h>
 #include <asm/cputable.h>
@@ -16,6 +18,8 @@
 
 static unsigned long dexcr_supported __ro_after_init = 0;
 
+static int spec_branch_hint_enable = -1;
+
 static int __init dexcr_init(void)
 {
 	if (!early_cpu_has_feature(CPU_FTR_ARCH_31))
@@ -37,6 +41,35 @@ static int __init dexcr_init(void)
 }
 early_initcall(dexcr_init);
 
+unsigned long get_thread_dexcr(struct thread_struct const *thread)
+{
+	unsigned long dexcr = thread->dexcr_enabled;
+
+	/* 
+	 * spec_branch_hint_enable may be written to concurrently via sysctl.
+	 * The sysctl handler is careful to use WRITE_ONCE, so we avoid
+	 * tearing/different values with READ_ONCE 
+	 */
+	switch (READ_ONCE(spec_branch_hint_enable)) {
+	case 0:
+		dexcr &= ~DEXCR_PR_SBHE;
+		break;
+	case 1:
+		dexcr |= DEXCR_PR_SBHE;
+		break;
+	}
+
+	return dexcr;
+}
+
+static void update_dexcr_on_cpu(void *_info)
+{
+	/* ensure the spec_branch_hint_enable write propagated to this CPU */
+	smp_mb();
+
+	mtspr(SPRN_DEXCR, get_thread_dexcr(&current->thread));
+}
+
 static int prctl_to_aspect(unsigned long which, unsigned int *aspect)
 {
 	switch (which) {
@@ -126,3 +159,55 @@ int set_dexcr_prctl(struct task_struct *task, unsigned long which, unsigned long
 
 	return 0;
 }
+
+#ifdef CONFIG_SYSCTL
+
+static const int min_sysctl_val = -1;
+
+static int sysctl_dexcr_sbhe_handler(struct ctl_table *table, int write,
+				     void *buf, size_t *lenp, loff_t *ppos)
+{
+	int err = 0;
+	int prev;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (!cpu_has_feature(CPU_FTR_DEXCR_SBHE))
+		return -ENODEV;
+
+	prev = READ_ONCE(spec_branch_hint_enable);
+	
+	err = proc_dointvec_minmax(table, write, buf, lenp, ppos);
+	if (err)
+		return err;
+
+	if (spec_branch_hint_enable != prev && write)
+		on_each_cpu(update_dexcr_on_cpu, NULL, 1);
+
+	return 0;
+}
+
+static struct ctl_table dexcr_sbhe_ctl_table[] = {
+	{
+		.procname	= "speculative_branch_hint_enable",
+		.data		= &spec_branch_hint_enable,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= sysctl_dexcr_sbhe_handler,
+		.extra1		= (void *)&min_sysctl_val,
+		.extra2		= SYSCTL_ONE,
+	}
+};
+
+static int __init register_dexcr_aspects_sysctl(void)
+{
+	if (!early_cpu_has_feature(CPU_FTR_DEXCR_SBHE))
+		return -ENODEV;
+
+	register_sysctl("kernel", dexcr_sbhe_ctl_table);
+	return 0;
+}
+device_initcall(register_dexcr_aspects_sysctl);
+
+#endif /* CONFIG_SYSCTL */
-- 
2.41.0