[PATCH v6 28/30] arm64: kpkeys: Batch KPKEYS_LVL_PGTABLES switches

Kevin Brodsky
2026-02-27T17:57:58Z

The kpkeys_hardened_pgtables feature currently switches kpkeys level
in every helper that writes to page tables, such as set_pte(). With
kpkeys implemented using POE, this entails a pair of ISBs whenever
such helper is called.

A simple way to reduce this overhead is to make use of the lazy MMU
mode. We amend the kpkeys_hardened_pgtables guard so that no level
switch (i.e. POR_EL1 update) is issued while the lazy MMU mode is
active. Instead, we switch to KPKEYS_LVL_PGTABLES when entering the
lazy MMU mode, and restore the previous level when exiting it.

Restoring the previous kpkeys level requires storing the original
value of POR_EL1 somewhere. This is a full 64-bit value so we cannot
simply use a TIF flag. There is no straightforward way to reuse
current->thread.por_el1 for that purpose - this is where the current
value of POR_EL1 is stored on a context switch, i.e. the value
corresponding to KPKEYS_LVL_PGTABLES inside a lazy_mmu section.
Instead, we add a new member to thread_struct to hold that value
temporarily. This isn't optimal as that member is unused outside of
lazy MMU sections, but it is the simplest option. Nesting of
sections is not a concern as arch_{enter,leave}_lazy_mmu_mode() are
not called in inner sections (nor do we need to do anything there).

A further optimisation this patch makes is to merge the ISBs when
exiting lazy_mmu mode. That is, if an ISB is going to be issued by
emit_pte_barriers() because kernel pgtables were modified in the
lazy MMU section, we skip the ISB after restoring POR_EL1. This is
done by checking TIF_LAZY_MMU_PENDING and ensuring that POR_EL1 is
restored before emit_pte_barriers() is called.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
arch/arm64/include/asm/pgtable.h | 50 +++++++++++++++++++++++++++---
arch/arm64/include/asm/processor.h | 1 +
2 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 8c85e23223da..556de0a4537e 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -43,10 +43,44 @@

#ifdef CONFIG_KPKEYS_HARDENED_PGTABLES
KPKEYS_GUARD_COND(kpkeys_hardened_pgtables, KPKEYS_LVL_PGTABLES,
- kpkeys_hardened_pgtables_enabled())
-#else
+ kpkeys_hardened_pgtables_enabled() &&
+ !is_lazy_mmu_mode_active())
+
+static void kpkeys_lazy_mmu_enter(void)
+{
+ if (!kpkeys_hardened_pgtables_enabled())
+ return;
+
+ current->thread.por_el1_lazy_mmu = kpkeys_set_level(KPKEYS_LVL_PGTABLES);
+}
+
+static void kpkeys_lazy_mmu_exit(void)
+{
+ u64 saved_por_el1;
+
+ if (!kpkeys_hardened_pgtables_enabled())
+ return;
+
+ saved_por_el1 = current->thread.por_el1_lazy_mmu;
+
+ /*
+ * We skip any barrier if TIF_LAZY_MMU_PENDING is set:
+ * emit_pte_barriers() will issue an ISB just after this function
+ * returns.
+ */
+ if (test_thread_flag(TIF_LAZY_MMU_PENDING))
+ __kpkeys_set_pkey_reg_nosync(saved_por_el1);
+ else
+ arch_kpkeys_restore_pkey_reg(saved_por_el1);
+}
+#else /* CONFIG_KPKEYS_HARDENED_PGTABLES */
KPKEYS_GUARD_NOOP(kpkeys_hardened_pgtables)
-#endif
+
+static void kpkeys_lazy_mmu_enter(void) {}
+static void kpkeys_lazy_mmu_exit(void) {}
+#endif /* CONFIG_KPKEYS_HARDENED_PGTABLES */
+
+

static inline void emit_pte_barriers(void)
{
@@ -79,7 +113,10 @@ static inline void queue_pte_barriers(void)
}
}

-static inline void arch_enter_lazy_mmu_mode(void) {}
+static inline void arch_enter_lazy_mmu_mode(void)
+{
+ kpkeys_lazy_mmu_enter();
+}

static inline void arch_flush_lazy_mmu_mode(void)
{
@@ -89,6 +126,11 @@ static inline void arch_flush_lazy_mmu_mode(void)

static inline void arch_leave_lazy_mmu_mode(void)
{
+ /*
+ * The ordering should be preserved to allow kpkeys_lazy_mmu_exit()
+ * to skip any barrier when TIF_LAZY_MMU_PENDING is set.
+ */
+ kpkeys_lazy_mmu_exit();
arch_flush_lazy_mmu_mode();
}

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 6095322343fc..c3a86ddce637 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -193,6 +193,7 @@ struct thread_struct {
u64 tpidr2_el0;
u64 por_el0;
u64 por_el1;
+ u64 por_el1_lazy_mmu;
#ifdef CONFIG_ARM64_GCS
unsigned int gcs_el0_mode;
unsigned int gcs_el0_locked;
--
2.51.2