[PATCH 2/4] x86/bus_lock: Set rate limit for bus lock

[Fenghua Yu]

A bus lock can be thousands of cycles slower than atomic operation within
one cache line. It also disrupts performance on other cores. Malicious
users may generate multiple bus locks to degrade the whole system
performance.

To mitigate the issue, the kernel can set a system wide rate limit for
bus locks via a kernel parameter:
	split_lock_detect=ratelimit:N

When the system detects bus locks at a rate higher than N/sec (where
N can be set by the kernel boot argument in the range [1..1000]) any
task triggering a bus lock will be forced to sleep for at least 20ms
until the overall system rate of bus locks drops below the threshold.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 arch/x86/kernel/cpu/intel.c | 43 +++++++++++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index fe0bec14d7ec..149c4d33e8c4 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -10,6 +10,7 @@
 #include <linux/thread_info.h>
 #include <linux/init.h>
 #include <linux/uaccess.h>
+#include <linux/delay.h>
 
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
@@ -41,6 +42,7 @@ enum split_lock_detect_state {
 	sld_off = 0,
 	sld_warn,
 	sld_fatal,
+	sld_ratelimit,
 };
 
 /*
@@ -997,13 +999,31 @@ static const struct {
 	{ "off",	sld_off   },
 	{ "warn",	sld_warn  },
 	{ "fatal",	sld_fatal },
+	{ "ratelimit:", sld_ratelimit },
 };
 
+static struct ratelimit_state bld_ratelimit;
+
 static inline bool match_option(const char *arg, int arglen, const char *opt)
 {
-	int len = strlen(opt);
+	int len = strlen(opt), ratelimit;
+
+	if (strncmp(arg, opt, len))
+		return false;
+
+	/*
+	 * Min ratelimit is 1 bus lock/sec.
+	 * Max ratelimit is 1000 bus locks/sec.
+	 */
+	if (sscanf(arg, "ratelimit:%d", &ratelimit) == 1 &&
+	    ratelimit > 0 && ratelimit <= 1000) {
+		ratelimit_state_init(&bld_ratelimit, HZ, ratelimit);
+		ratelimit_set_flags(&bld_ratelimit, RATELIMIT_MSG_ON_RELEASE);
 
-	return len == arglen && !strncmp(arg, opt, len);
+		return true;
+	}
+
+	return len == arglen;
 }
 
 static bool split_lock_verify_msr(bool on)
@@ -1082,6 +1102,15 @@ static void sld_update_msr(bool on)
 
 static void split_lock_init(void)
 {
+	/*
+	 * #DB for bus lock handles ratelimit and #AC for split lock is
+	 * disabled.
+	 */
+	if (sld_state == sld_ratelimit) {
+		split_lock_verify_msr(false);
+		return;
+	}
+
 	if (cpu_model_supports_sld)
 		split_lock_verify_msr(sld_state != sld_off);
 }
@@ -1154,6 +1183,12 @@ void handle_bus_lock(struct pt_regs *regs)
 	switch (sld_state) {
 	case sld_off:
 		break;
+	case sld_ratelimit:
+		/* Enforce no more than bld_ratelimit bus locks/sec. */
+		while (!__ratelimit(&bld_ratelimit))
+			msleep(20);
+		/* Warn on the bus lock. */
+		fallthrough;
 	case sld_warn:
 		pr_warn_ratelimited("#DB: %s/%d took a bus_lock trap at address: 0x%lx\n",
 				    current->comm, current->pid, regs->ip);
@@ -1259,6 +1294,10 @@ static void sld_state_show(void)
 				" from non-WB" : "");
 		}
 		break;
+	case sld_ratelimit:
+		if (boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT))
+			pr_info("#DB: setting system wide bus lock rate limit to %u/sec\n", bld_ratelimit.burst);
+		break;
 	}
 }
 
-- 
2.31.1