linux-kernel - [PATCH] x86: Optimize load_mm_cr4 to load_mm_cr4

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Date:   Tue, 18 Jun 2019 09:32:11 +0200
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH] x86: Optimize load_mm_cr4 to load_mm_cr4_irqsoff

From: Jan Kiszka <jan.kiszka@...mens.com>

We only call load_mm_cr4 with interrupts disabled:
 - switch_mm_irqs_off
 - refresh_pce (on_each_cpu callback)

Thus, we can avoid disabling interrupts again in cr4_set/clear_bits.

Instead, provide cr4_set/clear_bits_irqsoffs and call those helpers from
load_mm_cr4_irqsoff. The renaming in combination with the checks
in __cr4_set shall ensure that any changes in the boundary conditions of
the invocations will be detected.

Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
---

Found while porting Xenomai with its virtualized interrupt
infrastructure to a newer kernel.

 arch/x86/events/core.c             |  2 +-
 arch/x86/include/asm/mmu_context.h |  8 ++++----
 arch/x86/include/asm/tlbflush.h    | 30 +++++++++++++++++++++++-------
 arch/x86/mm/tlb.c                  |  2 +-
 4 files changed, 29 insertions(+), 13 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index f315425d8468..78a3fba28c62 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2161,7 +2161,7 @@ static int x86_pmu_event_init(struct perf_event *event)
 
 static void refresh_pce(void *ignored)
 {
-	load_mm_cr4(this_cpu_read(cpu_tlbstate.loaded_mm));
+	load_mm_cr4_irqsoff(this_cpu_read(cpu_tlbstate.loaded_mm));
 }
 
 static void x86_pmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 9024236693d2..16ae821483c8 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -28,16 +28,16 @@ static inline void paravirt_activate_mm(struct mm_struct *prev,
 
 DECLARE_STATIC_KEY_FALSE(rdpmc_always_available_key);
 
-static inline void load_mm_cr4(struct mm_struct *mm)
+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))
-		cr4_set_bits(X86_CR4_PCE);
+		cr4_set_bits_irqsoff(X86_CR4_PCE);
 	else
-		cr4_clear_bits(X86_CR4_PCE);
+		cr4_clear_bits_irqsoff(X86_CR4_PCE);
 }
 #else
-static inline void load_mm_cr4(struct mm_struct *mm) {}
+static inline void load_mm_cr4_irqsoff(struct mm_struct *mm) {}
 #endif
 
 #ifdef CONFIG_MODIFY_LDT_SYSCALL
diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index dee375831962..6f66d841262d 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -290,26 +290,42 @@ static inline void __cr4_set(unsigned long cr4)
 }
 
 /* Set in this cpu's CR4. */
-static inline void cr4_set_bits(unsigned long mask)
+static inline void cr4_set_bits_irqsoff(unsigned long mask)
 {
-	unsigned long cr4, flags;
+	unsigned long cr4;
 
-	local_irq_save(flags);
 	cr4 = this_cpu_read(cpu_tlbstate.cr4);
 	if ((cr4 | mask) != cr4)
 		__cr4_set(cr4 | mask);
-	local_irq_restore(flags);
 }
 
 /* Clear in this cpu's CR4. */
-static inline void cr4_clear_bits(unsigned long mask)
+static inline void cr4_clear_bits_irqsoff(unsigned long mask)
 {
-	unsigned long cr4, flags;
+	unsigned long cr4;
 
-	local_irq_save(flags);
 	cr4 = this_cpu_read(cpu_tlbstate.cr4);
 	if ((cr4 & ~mask) != cr4)
 		__cr4_set(cr4 & ~mask);
+}
+
+/* Set in this cpu's CR4. */
+static inline void cr4_set_bits(unsigned long mask)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	cr4_set_bits_irqsoff(mask);
+	local_irq_restore(flags);
+}
+
+/* Clear in this cpu's CR4. */
+static inline void cr4_clear_bits(unsigned long mask)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	cr4_clear_bits_irqsoff(mask);
 	local_irq_restore(flags);
 }
 
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 91f6db92554c..8fc1eaa55b6e 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -440,7 +440,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 	this_cpu_write(cpu_tlbstate.loaded_mm_asid, new_asid);
 
 	if (next != real_prev) {
-		load_mm_cr4(next);
+		load_mm_cr4_irqsoff(next);
 		switch_ldt(real_prev, next);
 	}
 }
-- 
2.16.4