Re: [PATCH bpf-next 8/9] bpf: Introduce bpf register BPF_REG_PARAMS

[Yonghong Song <yonghong.song@linux.dev>]

On 4/21/26 3:10 PM, Alexei Starovoitov wrote:
> On Tue Apr 21, 2026 at 10:20 AM PDT, Yonghong Song wrote:
>>   
>>   /* Kernel hidden auxiliary/helper register. */
>> -#define BPF_REG_AX		MAX_BPF_REG
>> -#define MAX_BPF_EXT_REG		(MAX_BPF_REG + 1)
>> +#define BPF_REG_PARAMS		MAX_BPF_REG
>> +#define BPF_REG_AX		(MAX_BPF_REG + 1)
>> +#define MAX_BPF_EXT_REG		(MAX_BPF_REG + 2)
> ...
>
>> --- a/tools/testing/selftests/bpf/progs/verifier_bpf_fastcall.c
>> +++ b/tools/testing/selftests/bpf/progs/verifier_bpf_fastcall.c
>> @@ -630,13 +630,13 @@ __xlated("...")
>>   __xlated("4: r0 = &(void __percpu *)(r0)")
>>   __xlated("...")
>>   /* may_goto expansion starts */
>> -__xlated("6: r11 = *(u64 *)(r10 -24)")
>> -__xlated("7: if r11 == 0x0 goto pc+6")
>> -__xlated("8: r11 -= 1")
>> -__xlated("9: if r11 != 0x0 goto pc+2")
>> -__xlated("10: r11 = -24")
>> +__xlated("6: r12 = *(u64 *)(r10 -24)")
>> +__xlated("7: if r12 == 0x0 goto pc+6")
>> +__xlated("8: r12 -= 1")
>> +__xlated("9: if r12 != 0x0 goto pc+2")
>> +__xlated("10: r12 = -24")
> maybe shift it to r15 right away, so we don't need to touch this code
> if/when true r12 is introduced?

We can do this. Do you think the following is okay:

diff --git a/include/linux/filter.h b/include/linux/filter.h
index b77d0b06db6e..fe7b6b943ea4 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -59,9 +59,9 @@ struct ctl_table_header;
  
  /* Kernel hidden auxiliary/helper register. */
  #define BPF_REG_PARAMS         MAX_BPF_REG
-#define BPF_REG_AX             (MAX_BPF_REG + 1)
-#define MAX_BPF_EXT_REG                (MAX_BPF_REG + 2)
+#define MAX_BPF_EXT_REG                (MAX_BPF_REG + 1)
  #define MAX_BPF_JIT_REG                MAX_BPF_EXT_REG
+#define BPF_REG_AX             (MAX_BPF_REG + 4)
  
  /* unused opcode to mark special call to bpf_tail_call() helper */
  #define BPF_TAIL_CALL  0xf0
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index ae10b9ca018d..3ad286ef3085 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1299,8 +1299,8 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
         u32 imm_rnd = get_random_u32();
         s16 off;
  
-       BUILD_BUG_ON(BPF_REG_PARAMS + 2 != MAX_BPF_JIT_REG);
-       BUILD_BUG_ON(BPF_REG_AX + 1 != MAX_BPF_JIT_REG);
+       BUILD_BUG_ON(BPF_REG_PARAMS + 1 != MAX_BPF_JIT_REG);
+       BUILD_BUG_ON(BPF_REG_AX != MAX_BPF_JIT_REG + 3);
  
         /* Constraints on AX register:
          *