linux-kernel - [PATCH] Immediate enforcement of /sys/devices/cpu/rdpmc set to 0.

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Message-Id: <20191125054838.137615-1-asteinhauser@google.com>
Date:   Sun, 24 Nov 2019 21:48:38 -0800
From:   Anthony Steinhauser <asteinhauser@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        x86@...nel.org, Anthony Steinhauser <asteinhauser@...gle.com>
Subject: [PATCH] Immediate enforcement of /sys/devices/cpu/rdpmc set to 0.

When you successfully write 0 to /sys/devices/cpu/rdpmc, the RDPMC
instruction should be disabled unconditionally and immediately (after you
close the SYSFS file) by the documentation.
Instead, in the current implementation the PMU must be reloaded which
happens only eventually some time in the future. Only after that the RDPMC
instruction becomes disabled (on ring 3) on the respective core.
This change makes the treatment of the 0 value as blocking and as
unconditional as the current treatment of the 2 value, only the CR4.PCE
bit is naturally set to false instead of true.

Signed-off-by: Anthony Steinhauser <asteinhauser@...gle.com>
---
 arch/x86/events/core.c             | 18 ++++++++++++------
 arch/x86/include/asm/mmu_context.h |  4 +++-
 2 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 7b21455d7504..0531092fe8f3 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -49,6 +49,7 @@ DEFINE_PER_CPU(struct cpu_hw_events, cpu_hw_events) = {
 	.enabled = 1,
 };
 
+DEFINE_STATIC_KEY_FALSE(rdpmc_never_available_key);
 DEFINE_STATIC_KEY_FALSE(rdpmc_always_available_key);
 
 u64 __read_mostly hw_cache_event_ids
@@ -2181,21 +2182,26 @@ static ssize_t set_attr_rdpmc(struct device *cdev,
 	if (x86_pmu.attr_rdpmc_broken)
 		return -ENOTSUPP;
 
-	if ((val == 2) != (x86_pmu.attr_rdpmc == 2)) {
+	if (val != x86_pmu.attr_rdpmc) {
 		/*
-		 * Changing into or out of always available, aka
-		 * perf-event-bypassing mode.  This path is extremely slow,
+		 * Changing into or out of never available or always available,
+		 * aka perf-event-bypassing mode. This path is extremely slow,
 		 * but only root can trigger it, so it's okay.
 		 */
+		if (val == 0)
+			static_branch_inc(&rdpmc_never_available_key);
+		else if (x86_pmu.attr_rdpmc == 0)
+			static_branch_dec(&rdpmc_never_available_key);
+
 		if (val == 2)
 			static_branch_inc(&rdpmc_always_available_key);
-		else
+		else if (x86_pmu.attr_rdpmc == 2)
 			static_branch_dec(&rdpmc_always_available_key);
+
 		on_each_cpu(refresh_pce, NULL, 1);
+		x86_pmu.attr_rdpmc = val;
 	}
 
-	x86_pmu.attr_rdpmc = val;
-
 	return count;
 }
 
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 16ae821483c8..bd60bbd2f959 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -26,12 +26,14 @@ static inline void paravirt_activate_mm(struct mm_struct *prev,
 
 #ifdef CONFIG_PERF_EVENTS
 
+DECLARE_STATIC_KEY_FALSE(rdpmc_never_available_key);
 DECLARE_STATIC_KEY_FALSE(rdpmc_always_available_key);
 
 static inline void load_mm_cr4_irqsoff(struct mm_struct *mm)
 {
 	if (static_branch_unlikely(&rdpmc_always_available_key) ||
-	    atomic_read(&mm->context.perf_rdpmc_allowed))
+	    (!static_branch_unlikely(&rdpmc_never_available_key) &&
+	     atomic_read(&mm->context.perf_rdpmc_allowed)))
 		cr4_set_bits_irqsoff(X86_CR4_PCE);
 	else
 		cr4_clear_bits_irqsoff(X86_CR4_PCE);
-- 
2.24.0.432.g9d3f5f5b63-goog