This patch enables support for PREEMPT_DYNAMIC on arm64, allowing the
preemption model to be chosen at boot time.

Specifically, this patch selects HAVE_PREEMPT_DYNAMIC_KEY, so that each
preemption function is an out-of-line call with an early return
depending upon a static key. This leaves almost all the codegen up to
the compiler, and side-steps a number of pain points with static calls
(e.g. interaction with CFI schemes). This should have no worse overhead
than using non-inline static calls, as those use out-of-line trampolines
with early returns.

For example, the dynamic_cond_resched() wrapper looks as follows (with
the first `B` being replaced with a `NOP` when the function is
disabled):

| <dynamic_cond_resched>:
|        bti     c
|        b       <dynamic_cond_resched+0x10>
|        mov     w0, #0x0                        // #0
|        ret
|        mrs     x0, sp_el0
|        ldr     x0, [x0, #8]
|        cbnz    x0, <dynamic_cond_resched+0x8>
|        paciasp
|        stp     x29, x30, [sp, #-16]!
|        mov     x29, sp
|        bl      <preempt_schedule_common>
|        mov     w0, #0x1                        // #1
|        ldp     x29, x30, [sp], #16
|        autiasp
|        ret

... compared to the regular form of the function:

| <__cond_resched>:
|        bti     c
|        mrs     x0, sp_el0
|        ldr     x1, [x0, #8]
|        cbz     x1, <__cond_resched+0x18>
|        mov     w0, #0x0                        // #0
|        ret
|        paciasp
|        stp     x29, x30, [sp, #-16]!
|        mov     x29, sp
|        bl      <preempt_schedule_common>
|        mov     w0, #0x1                        // #1
|        ldp     x29, x30, [sp], #16
|        autiasp
|        ret

Since arm64 does not yet use the generic entry code, we must define our
own `sk_dynamic_irqentry_exit_cond_resched`, which will be
enabled/disabled by the common code in kernel/sched/core.c. All other
preemption functions and associated static keys are defined there.

Note that PREEMPT_DYNAMIC is `def bool y`, so this will default to
enabled.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <redacted>
Cc: Will Deacon <will@kernel.org>
---
 arch/arm64/Kconfig               |  1 +
 arch/arm64/include/asm/preempt.h | 16 ++++++++++++++--
 arch/arm64/kernel/entry-common.c | 10 +++++++++-
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index f2b5a4abef21..3831d922a81d 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -192,6 +192,7 @@ config ARM64
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
 	select HAVE_FUNCTION_ARG_ACCESS_API

diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
index e83f0982b99c..73623f719d7e 100644
--- a/arch/arm64/include/asm/preempt.h
+++ b/arch/arm64/include/asm/preempt.h

@@ -80,10 +80,22 @@ static inline bool should_resched(int preempt_offset)
 }
 
 #ifdef CONFIG_PREEMPTION
+#ifdef CONFIG_PREEMPT_DYNAMIC
+
+DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+void dynamic_preempt_schedule(void);
+#define __preempt_schedule()		dynamic_preempt_schedule()
+void dynamic_preempt_schedule_notrace(void);
+#define __preempt_schedule_notrace()	dynamic_preempt_schedule_notrace()
+
+#else
+
 void preempt_schedule(void);
-#define __preempt_schedule() preempt_schedule()
+#define __preempt_schedule()		preempt_schedule()
 void preempt_schedule_notrace(void);
-#define __preempt_schedule_notrace() preempt_schedule_notrace()
+#define __preempt_schedule_notrace()	preempt_schedule_notrace()
+
+#endif
 #endif /* CONFIG_PREEMPTION */
 
 #endif /* __ASM_PREEMPT_H */

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 2c639b6b676d..675352ec1368 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c

@@ -220,9 +220,17 @@ static void noinstr arm64_exit_el1_dbg(struct pt_regs *regs)
 		lockdep_hardirqs_on(CALLER_ADDR0);
 }
 
+#ifdef CONFIG_PREEMPT_DYNAMIC
+DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+#define need_irq_preemption() \
+	(static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
+#else
+#define need_irq_preemption()	(IS_ENABLED(CONFIG_PREEMPTION))
+#endif
+
 static void __sched arm64_preempt_schedule_irq(void)
 {
-	if (!IS_ENABLED(CONFIG_PREEMPTION))
+	if (!need_irq_preemption())
 		return;
 
 	/*

-- 
2.30.2


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