[PATCH bpf-next v2 1/3] bpf, arm64: Map BPF_REG_0 to x8 instead of x7

Puranjay Mohan
2026-04-27T23:48:21Z

Move the BPF return value register from x7 to x8, freeing x7 for use
as an argument register. AAPCS64 designates x8 as the indirect result
location register; it is caller-saved and not used for argument
passing, making it a suitable home for BPF_REG_0.

This is a prerequisite for stack argument support, which needs x5-x7
to pass arguments 6-8 to native kfuncs following the AAPCS64 calling
convention.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
arch/arm64/net/bpf_jit_comp.c | 4 ++--
arch/arm64/net/bpf_timed_may_goto.S | 8 ++++----
.../testing/selftests/bpf/progs/verifier_jit_inline.c | 2 +-
tools/testing/selftests/bpf/progs/verifier_ldsx.c | 6 +++---
.../selftests/bpf/progs/verifier_private_stack.c | 10 +++++-----
5 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 0816c40fc7af..085e650662e3 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -47,7 +47,7 @@
/* Map BPF registers to A64 registers */
static const int bpf2a64[] = {
/* return value from in-kernel function, and exit value from eBPF */
- [BPF_REG_0] = A64_R(7),
+ [BPF_REG_0] = A64_R(8),
/* arguments from eBPF program to in-kernel function */
[BPF_REG_1] = A64_R(0),
[BPF_REG_2] = A64_R(1),
@@ -1048,7 +1048,7 @@ static void build_epilogue(struct jit_ctx *ctx, bool was_classic)
/* Restore FP/LR registers */
emit(A64_POP(A64_FP, A64_LR, A64_SP), ctx);

- /* Move the return value from bpf:r0 (aka x7) to x0 */
+ /* Move the return value from bpf:r0 (aka x8) to x0 */
emit(A64_MOV(1, A64_R(0), r0), ctx);

/* Authenticate lr */
diff --git a/arch/arm64/net/bpf_timed_may_goto.S b/arch/arm64/net/bpf_timed_may_goto.S
index 894cfcd7b241..a9a802711a7f 100644
--- a/arch/arm64/net/bpf_timed_may_goto.S
+++ b/arch/arm64/net/bpf_timed_may_goto.S
@@ -8,8 +8,8 @@ SYM_FUNC_START(arch_bpf_timed_may_goto)
stp x29, x30, [sp, #-64]!
mov x29, sp

- /* Save BPF registers R0 - R5 (x7, x0-x4)*/
- stp x7, x0, [sp, #16]
+ /* Save BPF registers R0 - R5 (x8, x0-x4)*/
+ stp x8, x0, [sp, #16]
stp x1, x2, [sp, #32]
stp x3, x4, [sp, #48]

@@ -28,8 +28,8 @@ SYM_FUNC_START(arch_bpf_timed_may_goto)
/* BPF_REG_AX(x9) will be stored into count, so move return value to it. */
mov x9, x0

- /* Restore BPF registers R0 - R5 (x7, x0-x4) */
- ldp x7, x0, [sp, #16]
+ /* Restore BPF registers R0 - R5 (x8, x0-x4) */
+ ldp x8, x0, [sp, #16]
ldp x1, x2, [sp, #32]
ldp x3, x4, [sp, #48]

diff --git a/tools/testing/selftests/bpf/progs/verifier_jit_inline.c b/tools/testing/selftests/bpf/progs/verifier_jit_inline.c
index 4ea254063646..885ff69a3a62 100644
--- a/tools/testing/selftests/bpf/progs/verifier_jit_inline.c
+++ b/tools/testing/selftests/bpf/progs/verifier_jit_inline.c
@@ -9,7 +9,7 @@ __success __retval(0)
__arch_x86_64
__jited(" addq %gs:{{.*}}, %rax")
__arch_arm64
-__jited(" mrs x7, SP_EL0")
+__jited(" mrs x8, SP_EL0")
int inline_bpf_get_current_task(void)
{
bpf_get_current_task();
diff --git a/tools/testing/selftests/bpf/progs/verifier_ldsx.c b/tools/testing/selftests/bpf/progs/verifier_ldsx.c
index c8494b682c31..c814e82a7242 100644
--- a/tools/testing/selftests/bpf/progs/verifier_ldsx.c
+++ b/tools/testing/selftests/bpf/progs/verifier_ldsx.c
@@ -274,11 +274,11 @@ __jited("movslq 0x10(%rdi,%r12), %r15")
__jited("movswq 0x18(%rdi,%r12), %r15")
__jited("movsbq 0x20(%rdi,%r12), %r15")
__arch_arm64
-__jited("add x11, x7, x28")
+__jited("add x11, x8, x28")
__jited("ldrsw x21, [x11, #0x10]")
-__jited("add x11, x7, x28")
+__jited("add x11, x8, x28")
__jited("ldrsh x21, [x11, #0x18]")
-__jited("add x11, x7, x28")
+__jited("add x11, x8, x28")
__jited("ldrsb x21, [x11, #0x20]")
__jited("add x11, x0, x28")
__jited("ldrsw x22, [x11, #0x10]")
diff --git a/tools/testing/selftests/bpf/progs/verifier_private_stack.c b/tools/testing/selftests/bpf/progs/verifier_private_stack.c
index 646e8ef82051..c5078face38d 100644
--- a/tools/testing/selftests/bpf/progs/verifier_private_stack.c
+++ b/tools/testing/selftests/bpf/progs/verifier_private_stack.c
@@ -170,12 +170,12 @@ __jited(" mrs x10, TPIDR_EL{{[0-1]}}")
__jited(" add x27, x27, x10")
__jited(" add x25, x27, {{.*}}")
__jited(" bl 0x{{.*}}")
-__jited(" mov x7, x0")
+__jited(" mov x8, x0")
__jited(" mov x0, #0x2a")
__jited(" str x0, [x27]")
__jited(" bl 0x{{.*}}")
-__jited(" mov x7, x0")
-__jited(" mov x7, #0x0")
+__jited(" mov x8, x0")
+__jited(" mov x8, #0x0")
__jited(" ldp x25, x27, [sp], {{.*}}")
__naked void private_stack_callback(void)
{
@@ -220,7 +220,7 @@ __jited(" mov x0, #0x2a")
__jited(" str x0, [x27]")
__jited(" mov x0, #0x0")
__jited(" bl 0x{{.*}}")
-__jited(" mov x7, x0")
+__jited(" mov x8, x0")
__jited(" ldp x27, x28, [sp], #0x10")
int private_stack_exception_main_prog(void)
{
@@ -258,7 +258,7 @@ __jited(" add x25, x27, {{.*}}")
__jited(" mov x0, #0x2a")
__jited(" str x0, [x27]")
__jited(" bl 0x{{.*}}")
-__jited(" mov x7, x0")
+__jited(" mov x8, x0")
__jited(" ldp x27, x28, [sp], #0x10")
int private_stack_exception_sub_prog(void)
{
--
2.52.0