Re: [PATCH bpf-next 0/7] bpf: Inline IRQ enter/exit instructions

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Tue, Oct 27, 2026 at 09:28 AM Emil Tsalapatis <emil@etsalapatis.com> wrote:
> Add inlining for those functions. The inlining happens in two stages.
> First replace the call itself with a special BPF instruction denoting
> an IRQ disable-and save, or restore-and-enable. Those instructions
> are translated by the architecture-specific JIT into the corresponding
> operation.

No. That doesn't scale. We cannot add new insns for every to-be-inlined kfunc.
See bpf_jit_inlines_helper_call().

Like:

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 6a2fe3d..c1c9a1c 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -3495,6 +3495,17 @@ populate_extable:
 			}
 			if (!imm32)
 				return -EINVAL;
+			if (src_reg == BPF_PSEUDO_KFUNC_CALL && bpf_jit_inlines_kfunc_call(func)) {
+				if (func == (u8 *)bpf_local_irq_save) {
+					/* pushfq; pop [rdi]; cli */
+					EMIT4(0x9C, 0x8F, 0x07, 0xFA);
+				} else {
+					/* test dword ptr [rdi], X86_EFLAGS_IF; je 1f; sti; 1: */
+					EMIT2_off32(0xF7, 0x07, X86_EFLAGS_IF);
+					EMIT3(X86_JE, 1, 0xFB);
+				}
+				break;
+			}
 			if (src_reg == BPF_PSEUDO_KFUNC_CALL) {
 				fm = bpf_jit_find_kfunc_model(bpf_prog, insn);
 				if (!fm)
@@ -5116,6 +5127,14 @@ bool bpf_jit_supports_kfunc_call(void)
 	return true;
 }

+bool bpf_jit_inlines_kfunc_call(void *func)
+{
+	if (!IS_ENABLED(CONFIG_BPF_SYSCALL) || IS_ENABLED(CONFIG_TRACE_IRQFLAGS) ||
+	    cpu_feature_enabled(X86_FEATURE_XENPV))
+		return false;
+	return func == bpf_local_irq_save || func == bpf_local_irq_restore;
+}

...

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 648abe1..26b6a9d 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -5197,8 +5197,8 @@ BTF_ID_FLAGS(func, bpf_get_kmem_cache, KF_PERFMON)
 BTF_ID_FLAGS(func, bpf_iter_kmem_cache_new, KF_ITER_NEW | KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_iter_kmem_cache_next, KF_ITER_NEXT | KF_RET_NULL | KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_iter_kmem_cache_destroy, KF_ITER_DESTROY | KF_SLEEPABLE)
-BTF_ID_FLAGS(func, bpf_local_irq_save)
-BTF_ID_FLAGS(func, bpf_local_irq_restore)
+BTF_ID_FLAGS(func, bpf_local_irq_save, KF_FASTCALL)
+BTF_ID_FLAGS(func, bpf_local_irq_restore, KF_FASTCALL)

and corresponding bits in bpf_get_call_summary()...

but before doing any of that there gotta be performance numbers that it's worth doing.

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