[RFC PATCH v4 18/18] arm64: mm: Batch kpkeys level switches

[Kevin Brodsky]

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.

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.

Checking TIF_LAZY_MMU flag in all pgtable writers is currently
overkill, as lazy_mmu sections are only used at the lowest level of
page tables. In other words, set_pgd() (for instance) will never be
called with TIF_LAZY_MMU set. However, such higher-level helpers are
called relatively infrequently and the overhead of checking a TIF
flag is low. The flag is therefore checked in all cases for
simplicity's sake, just like in [1].

[1] https://lore.kernel.org/linux-mm/20250304150444.3788920-1-ryan.roberts@xxxxxxx/

Signed-off-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
---
 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 7929b79cd6b1..61dee76be515 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() &&
+		  !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)
 {
 	/*
@@ -83,6 +112,7 @@ static inline void arch_enter_lazy_mmu_mode(void)
 	VM_WARN_ON(test_thread_flag(TIF_LAZY_MMU));
 
 	set_thread_flag(TIF_LAZY_MMU);
+	kpkeys_lazy_mmu_enter();
 }
 
 static inline void arch_flush_lazy_mmu_mode(void)
@@ -93,6 +123,11 @@ 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();
 	clear_thread_flag(TIF_LAZY_MMU);
 }
diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 0afaf96ca699..14a4b483098d 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -186,6 +186,7 @@ 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