[PATCH bpf-next 2/2] bpf, riscv: inline bpf_get_current_task() and bpf_get_current_task_btf()

[Varun R Mallya <varunrmallya@gmail.com>]

On RISC-V, the current task pointer is stored in the thread pointer
register (tp). Emit a single `mv a5, tp` instead of a full helper
call for BPF_FUNC_get_current_task and BPF_FUNC_get_current_task_btf.

Register bpf_jit_inlines_helper_call() entries for both helpers so the
verifier treats them as inlined, and add the expected `mv a5, tp`
annotation to the riscv64 selftests.

The following show changes before and after this patch.

Before patch:

      auipc  t1,0x817a    # load upper PC-relative address
      jalr   -2004(t1)    # call bpf_get_current_task helper
      mv     a5,a0        # move return value to BPF_REG_0

After patch:

      mv     a5,tp        # directly: a5 = current (tp = thread pointer)

Benchmark (bpf_prog_test_run wrapping bpf_get_current_task in loop,
batch=100, 10s, QEMU RISC-V):

              | runs/sec  | helper-calls/sec | ns/call
 -------------+-----------+------------------+---------
 Before patch |   173,490 |       17,349,090 |      57
 After patch  |   320,497 |       32,049,780 |      31
 -------------+-----------+------------------+---------
 Improvement  |   +84.7%  |          +84.7%  |  -45.6%

Signed-off-by: Varun R Mallya <varunrmallya@gmail.com>
---
 arch/riscv/net/bpf_jit_comp64.c                         | 9 +++++++++
 tools/testing/selftests/bpf/progs/verifier_jit_inline.c | 2 ++
 2 files changed, 11 insertions(+)

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index 2f1109dbf105..e2c70c70cca8 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -1808,6 +1808,13 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			break;
 		}
 
+		/* Implement helper call to bpf_get_current_task/_btf() inline */
+		if (insn->src_reg == 0 && (insn->imm == BPF_FUNC_get_current_task ||
+					   insn->imm == BPF_FUNC_get_current_task_btf)) {
+			emit_mv(bpf_to_rv_reg(BPF_REG_0, ctx), RV_REG_TP, ctx);
+			break;
+		}
+
 		mark_call(ctx);
 		ret = bpf_jit_get_func_addr(ctx->prog, insn, extra_pass,
 					    &addr, &fixed_addr);
@@ -2138,6 +2145,8 @@ bool bpf_jit_inlines_helper_call(s32 imm)
 {
 	switch (imm) {
 	case BPF_FUNC_get_smp_processor_id:
+	case BPF_FUNC_get_current_task:
+	case BPF_FUNC_get_current_task_btf:
 		return true;
 	default:
 		return false;
diff --git a/tools/testing/selftests/bpf/progs/verifier_jit_inline.c b/tools/testing/selftests/bpf/progs/verifier_jit_inline.c
index 885ff69a3a62..76d80605ec7f 100644
--- a/tools/testing/selftests/bpf/progs/verifier_jit_inline.c
+++ b/tools/testing/selftests/bpf/progs/verifier_jit_inline.c
@@ -10,6 +10,8 @@ __arch_x86_64
 __jited("	addq	%gs:{{.*}}, %rax")
 __arch_arm64
 __jited("	mrs	x8, SP_EL0")
+__arch_riscv64
+__jited("	mv	a5, tp")
 int inline_bpf_get_current_task(void)
 {
 	bpf_get_current_task();
-- 
2.54.0