arm64: Support PREEMPT_DYNAMIC

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 when
enabled. When disabled, the first `B` is replaced with a `NOP`,
resulting in an early return.

| <dynamic_cond_resched>:
|        bti     c
|        b       <dynamic_cond_resched+0x10>     // or `nop`
|        mov     w0, #0x0
|        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
|        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
|        ret
|        paciasp
|        stp     x29, x30, [sp, #-16]!
|        mov     x29, sp
|        bl      <preempt_schedule_common>
|        mov     w0, #0x1
|        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.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20220214165216.2231574-8-mark.rutland@arm.com

1 files changed, 9 insertions, 1 deletions

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;
 
 	/*