[RFC PATCH v5 18/18] arm64: mm: Batch kpkeys level switches

Kevin Brodsky
2025-08-15T11:02:42Z

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, which has recently been adopted on arm64 to batch barriers
(DSB/ISB) when updating kernel pgtables [1]. Reusing the
TIF_LAZY_MMU flag introduced by this series, we amend the
kpkeys_hardened_pgtables guard so that no level switch (i.e. POR_EL1
update) is issued while that flag is set. Instead, we switch to
KPKEYS_LVL_PGTABLES when entering lazy_mmu mode, and restore the
previous level when exiting it. The optimisation is disabled while
in interrupt as POR_EL1 is reset on exception entry, i.e. switching
is not batched in that case.

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, but since lazy_mmu sections cannot nest, some
sort of thread-local variable would do the trick. 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.

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.

[1] https://lore.kernel.org/all/20250422081822.1836315-12-ryan.roberts@arm.com/

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---

Unfortunately lazy_mmu sections can in fact nest under certain
circumstances [2], which means that storing the original value of
POR_EL1 in thread_struct is not always safe.

I am working on modifying the lazy_mmu API to handle nesting gracefully,
which should also help with restoring POR_EL1 without using
thread_struct. See also the discussion in [3].

[2] https://lore.kernel.org/all/20250512150333.5589-1-ryan.roberts@arm.com/
[3] https://lore.kernel.org/all/20250606135654.178300-1-ryan.roberts@arm.com/t/#u

arch/arm64/include/asm/pgtable.h | 37 +++++++++++++++++++++++++++++-
arch/arm64/include/asm/processor.h | 1 +
2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 1694fb839854..35d15b9722e4 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -43,11 +43,40 @@

#ifdef CONFIG_KPKEYS_HARDENED_PGTABLES
KPKEYS_GUARD_COND(kpkeys_hardened_pgtables, KPKEYS_LVL_PGTABLES,
- kpkeys_hardened_pgtables_enabled())
+ kpkeys_hardened_pgtables_enabled() &&
+ (in_interrupt() || !test_thread_flag(TIF_LAZY_MMU)))
#else
KPKEYS_GUARD_NOOP(kpkeys_hardened_pgtables)
#endif

+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);
+}
+
static inline void emit_pte_barriers(void)
{
/*
@@ -107,6 +136,7 @@ static inline void arch_enter_lazy_mmu_mode(void)
return;

set_thread_flag(TIF_LAZY_MMU);
+ kpkeys_lazy_mmu_enter();
}

static inline void arch_flush_lazy_mmu_mode(void)
@@ -123,6 +153,11 @@ static inline void arch_leave_lazy_mmu_mode(void)
if (in_interrupt())
return;

+ /*
+ * 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();
clear_thread_flag(TIF_LAZY_MMU);
}
diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 9340e94a27f6..7b20eedfe2fe 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -188,6 +188,7 @@ 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;
--
2.47.0