Re: [PATCH bpf-next v7 1/2] bpf: Introduce jit_required flag and refactor kfunc path

[KaFai Wan <kafai.wan@linux.dev>]

On Thu, 2026-07-02 at 22:36 +0800, Tiezhu Yang wrote:
> Introduce a 'jit_required' bitfield flag at the end of the
> flags group in struct bpf_prog. This bit tracks whether a
> program strictly requires the JIT compiler.
> 
> Set this flag to 1 when a kfunc call is added at the end of
> bpf_add_kfunc_call().
> 
> In __bpf_prog_select_runtime(), check with fp->jit_required
> rather than bpf_prog_has_kfunc_call() to unify the logic.
> 
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Suggested-by: KaFai Wan <kafai.wan@linux.dev>
> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
> ---
>  include/linux/bpf.h   | 3 ++-
>  kernel/bpf/core.c     | 3 +--
>  kernel/bpf/verifier.c | 2 ++
>  3 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index ba09795e0bfd..4e2b059d71f3 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1880,7 +1880,8 @@ struct bpf_prog {
>  				call_get_func_ip:1, /* Do we call get_func_ip() */
>  				call_session_cookie:1, /* Do we call bpf_session_cookie() */
>  				tstamp_type_access:1, /* Accessed __sk_buff->tstamp_type */
> -				sleepable:1;	/* BPF program is sleepable */
> +				sleepable:1,	/* BPF program is sleepable */
> +				jit_required:1;	/* program strictly requires JIT compiler
> */

In v6 you're using 'u8 jit_required', I thought it would be 'u8 jit_required:1', so we use one byte
and left 3 byte hole for future use.

This version left 2 bytes hole, same as v5, but it can easily be misleading because of the u16 type,
since we used 17 bits. I prefer v5, it has better readability.

>  	enum bpf_prog_type	type;		/* Type of BPF program */
>  	enum bpf_attach_type	expected_attach_type; /* For some prog types */
>  	u32			len;		/* Number of filter blocks */
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 649cce41e13f..5fcd19ccb41a 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2619,8 +2619,7 @@ struct bpf_prog *__bpf_prog_select_runtime(struct bpf_verifier_env *env,
> struct
>  	if (fp->bpf_func)
>  		goto finalize;
>  
> -	if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) ||
> -	    bpf_prog_has_kfunc_call(fp))
> +	if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) || fp->jit_required)
>  		jit_needed = true;
>  
>  	if (!bpf_prog_select_interpreter(fp))
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 25aea4271cd0..f496b45b9da4 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2765,6 +2765,8 @@ int bpf_add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, u16
> offset)
>  	desc->func_model = func_model;
>  	sort(tab->descs, tab->nr_descs, sizeof(tab->descs[0]),
>  	     kfunc_desc_cmp_by_id_off, NULL);
> +
> +	env->prog->jit_required = 1;

That's ok, and I think Sashiko is right about this patch.

>  	return 0;
>  }
>  

-- 
Thanks,
KaFai