[PATCH bpf-next v1 10/14] bpf, powerpc: JIT arena kfunc argument rebasing

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Kfunc parameters annotated with __arena carry a 32-bit arena offset in a
BPF register. The kernel function expects a directly dereferenceable
kernel address, so the JIT must add the arena kernel mapping base before
making the call. Nullable parameters must preserve offset zero as NULL.

PowerPC64 already keeps the arena kernel base in r26 for arena memory
accesses. Reuse it in the kfunc ABI preparation path: zero-extend each
arena argument to 32 bits, skip the addition for nullable zero, and
otherwise add r26. Advertise the kfunc-argument capability on PowerPC64
so arena allocation kfuncs can move to suffix annotations without losing
PowerPC support.

Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Christophe Leroy <chleroy@kernel.org>
Cc: Naveen N. Rao <naveen@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 arch/powerpc/net/bpf_jit_comp.c   |  5 +++++
 arch/powerpc/net/bpf_jit_comp64.c | 18 +++++++++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index 7b07b43575f1..911088cbabb9 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -527,6 +527,11 @@ bool bpf_jit_supports_kfunc_call(void)
 	return IS_ENABLED(CONFIG_PPC64);
 }
 
+bool bpf_jit_supports_arena_kfunc_args(void)
+{
+	return IS_ENABLED(CONFIG_PPC64);
+}
+
 bool bpf_jit_supports_private_stack(void)
 {
 	return IS_ENABLED(CONFIG_PPC64);
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index fc9db691e820..fc235fbfbeb9 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -624,8 +624,24 @@ static int prepare_for_kfunc_call(const struct bpf_prog *fp, u32 *image,
 	for (i = 0; i < m->nr_args; i++) {
 		/* Note that BPF ABI only allows up to 5 args for kfuncs */
 		u32 reg = bpf_to_ppc(BPF_REG_1 + i), size = m->arg_size[i];
+		u8 flags = m->arg_flags[i];
 
-		if (!(m->arg_flags[i] & BTF_FMODEL_SIGNED_ARG)) {
+		if (flags & BTF_FMODEL_ARENA_ARG) {
+			if (WARN_ON_ONCE(!ctx->arena_vm_start))
+				return -1;
+
+			/* rN = kern_vm_start + (u32)rN */
+			if (zero_extend(image, ctx, reg, reg, 4))
+				return -1;
+			if (flags & BTF_FMODEL_NULLABLE_ARG) {
+				EMIT(PPC_RAW_CMPLDI(reg, 0));
+				PPC_BCC_CONST_SHORT(COND_EQ, 8);
+			}
+			EMIT(PPC_RAW_ADD(reg, reg, bpf_to_ppc(ARENA_VM_START)));
+			continue;
+		}
+
+		if (!(flags & BTF_FMODEL_SIGNED_ARG)) {
 			if (zero_extend(image, ctx, reg, reg, size))
 				return -1;
 		} else {
-- 
2.53.0