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 <bg...@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(¤t->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
