[PATCH v6 2/2] x86/tlb: skip redundant sync IPIs for native TLB flush

[Lance Yang <lance.yang@linux.dev>]

From: Lance Yang <lance.yang@linux.dev>

Enable the optimization introduced in the previous patch for x86.

Add pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast to track whether
flush_tlb_multi() sends real IPIs. Initialize it once in
native_pv_tlb_init() during boot.

On CONFIG_PARAVIRT systems, tlb_table_flush_implies_ipi_broadcast() reads
the pv_ops property. On non-PARAVIRT, it directly checks for INVLPGB.

PV backends (KVM, Xen, Hyper-V) typically have their own implementations
and don't call native_flush_tlb_multi() directly, so they cannot be trusted
to provide the IPI guarantees we need. They keep the property false.

Two-step plan as David suggested[1]:

Step 1 (this patch): Skip redundant sync when we're 100% certain the TLB
flush sent IPIs. INVLPGB is excluded because when supported, we cannot
guarantee IPIs were sent, keeping it clean and simple.

Step 2 (future work): Send targeted IPIs only to CPUs actually doing
software/lockless page table walks, benefiting all architectures.

Regarding Step 2, it obviously only applies to setups where Step 1 does
not apply: like x86 with INVLPGB or arm64.

[1] https://lore.kernel.org/linux-mm/bbfdf226-4660-4949-b17b-0d209ee4ef8c@kernel.org/

Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
 arch/x86/include/asm/paravirt_types.h |  5 +++++
 arch/x86/include/asm/smp.h            |  3 +++
 arch/x86/include/asm/tlb.h            | 16 +++++++++++++++-
 arch/x86/include/asm/tlbflush.h       |  3 +++
 arch/x86/kernel/paravirt.c            | 20 ++++++++++++++++++++
 arch/x86/kernel/smpboot.c             |  1 +
 arch/x86/mm/tlb.c                     | 14 ++++++++++++++
 7 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index 9bcf6bce88f6..ec01268f2e3e 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -112,6 +112,11 @@ struct pv_mmu_ops {
 	void (*flush_tlb_multi)(const struct cpumask *cpus,
 				const struct flush_tlb_info *info);
 
+	/*
+	 * True if flush_tlb_multi() sends real IPIs to all target CPUs.
+	 */
+	bool flush_tlb_multi_implies_ipi_broadcast;
+
 	/* Hook for intercepting the destruction of an mm_struct. */
 	void (*exit_mmap)(struct mm_struct *mm);
 	void (*notify_page_enc_status_changed)(unsigned long pfn, int npages, bool enc);
diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index 84951572ab81..ef1fe0cc4c73 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -105,6 +105,9 @@ void native_smp_prepare_boot_cpu(void);
 void smp_prepare_cpus_common(void);
 void native_smp_prepare_cpus(unsigned int max_cpus);
 void native_smp_cpus_done(unsigned int max_cpus);
+
+void __init native_pv_tlb_init(void);
+
 int common_cpu_up(unsigned int cpunum, struct task_struct *tidle);
 int native_kick_ap(unsigned int cpu, struct task_struct *tidle);
 int native_cpu_disable(void);
diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
index 866ea78ba156..532578c5a2e7 100644
--- a/arch/x86/include/asm/tlb.h
+++ b/arch/x86/include/asm/tlb.h
@@ -5,10 +5,19 @@
 #define tlb_flush tlb_flush
 static inline void tlb_flush(struct mmu_gather *tlb);
 
+#define tlb_table_flush_implies_ipi_broadcast tlb_table_flush_implies_ipi_broadcast
+static inline bool tlb_table_flush_implies_ipi_broadcast(void);
+
 #include <asm-generic/tlb.h>
 #include <linux/kernel.h>
 #include <vdso/bits.h>
 #include <vdso/page.h>
+#include <asm/paravirt.h>
+
+static inline bool tlb_table_flush_implies_ipi_broadcast(void)
+{
+	return static_branch_likely(&tlb_ipi_broadcast_key);
+}
 
 static inline void tlb_flush(struct mmu_gather *tlb)
 {
@@ -20,7 +29,12 @@ static inline void tlb_flush(struct mmu_gather *tlb)
 		end = tlb->end;
 	}
 
-	flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables);
+	/*
+	 * Pass both freed_tables and unshared_tables so that lazy-TLB CPUs
+	 * also receive IPIs during unsharing page tables.
+	 */
+	flush_tlb_mm_range(tlb->mm, start, end, stride_shift,
+			   tlb->freed_tables || tlb->unshared_tables);
 }
 
 static inline void invlpg(unsigned long addr)
diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 5a3cdc439e38..a1b5efef3b90 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -5,6 +5,7 @@
 #include <linux/mm_types.h>
 #include <linux/mmu_notifier.h>
 #include <linux/sched.h>
+#include <linux/jump_label.h>
 
 #include <asm/barrier.h>
 #include <asm/processor.h>
@@ -18,6 +19,8 @@
 
 DECLARE_PER_CPU(u64, tlbstate_untag_mask);
 
+DECLARE_STATIC_KEY_FALSE(tlb_ipi_broadcast_key);
+
 void __flush_tlb_all(void);
 
 #define TLB_FLUSH_ALL	-1UL
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index a6ed52cae003..c8decadf16e0 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -154,6 +154,7 @@ struct paravirt_patch_template pv_ops = {
 	.mmu.flush_tlb_kernel	= native_flush_tlb_global,
 	.mmu.flush_tlb_one_user	= native_flush_tlb_one_user,
 	.mmu.flush_tlb_multi	= native_flush_tlb_multi,
+	.mmu.flush_tlb_multi_implies_ipi_broadcast = false,
 
 	.mmu.exit_mmap		= paravirt_nop,
 	.mmu.notify_page_enc_status_changed	= paravirt_nop,
@@ -221,3 +222,22 @@ NOKPROBE_SYMBOL(native_load_idt);
 
 EXPORT_SYMBOL(pv_ops);
 EXPORT_SYMBOL_GPL(pv_info);
+
+void __init native_pv_tlb_init(void)
+{
+	/*
+	 * If PV backend already set the property, respect it.
+	 * Otherwise, check if native TLB flush sends real IPIs to all target
+	 * CPUs (i.e., not using INVLPGB broadcast invalidation).
+	 */
+	if (pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast) {
+		static_branch_enable(&tlb_ipi_broadcast_key);
+		return;
+	}
+
+	if (pv_ops.mmu.flush_tlb_multi == native_flush_tlb_multi &&
+	    !cpu_feature_enabled(X86_FEATURE_INVLPGB)) {
+		pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast = true;
+		static_branch_enable(&tlb_ipi_broadcast_key);
+	}
+}
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 5cd6950ab672..3cdb04162843 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1167,6 +1167,7 @@ void __init native_smp_prepare_boot_cpu(void)
 		switch_gdt_and_percpu_base(me);
 
 	native_pv_lock_init();
+	native_pv_tlb_init();
 }
 
 void __init native_smp_cpus_done(unsigned int max_cpus)
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 621e09d049cb..7b1acfb97782 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -26,6 +26,8 @@
 
 #include "mm_internal.h"
 
+DEFINE_STATIC_KEY_FALSE(tlb_ipi_broadcast_key);
+
 #ifdef CONFIG_PARAVIRT
 # define STATIC_NOPV
 #else
@@ -1834,3 +1836,15 @@ static int __init create_tlb_single_page_flush_ceiling(void)
 	return 0;
 }
 late_initcall(create_tlb_single_page_flush_ceiling);
+
+#ifndef CONFIG_PARAVIRT
+void __init native_pv_tlb_init(void)
+{
+	/*
+	 * For non-PARAVIRT builds, check if native TLB flush sends real IPIs
+	 * (i.e., not using INVLPGB broadcast invalidation).
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_INVLPGB))
+		static_branch_enable(&tlb_ipi_broadcast_key);
+}
+#endif
-- 
2.49.0