linux-kernel - Re: [PATCH v2 1/6] powerpc64/bpf: Move tail_call

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Message-ID: <2c0588cd-f71b-40e5-a2cd-269d27e35abc@kernel.org>
Date: Thu, 15 Jan 2026 09:29:06 +0100
From: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>
To: adubey@...ux.ibm.com, bpf@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: hbathini@...ux.ibm.com, sachinpb@...ux.ibm.com, venkat88@...ux.ibm.com,
 andrii@...nel.org, eddyz87@...il.com, mykolal@...com, ast@...nel.org,
 daniel@...earbox.net, martin.lau@...ux.dev, song@...nel.org,
 yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org,
 sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, naveen@...nel.org,
 maddy@...ux.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
 memxor@...il.com, iii@...ux.ibm.com, shuah@...nel.org
Subject: Re: [PATCH v2 1/6] powerpc64/bpf: Move tail_call_cnt to bottom of
 stack frame



Le 14/01/2026 à 12:44, adubey@...ux.ibm.com a écrit :
> From: Abhishek Dubey <adubey@...ux.ibm.com>
> 
> In the conventional stack frame, the position of tail_call_cnt
> is after the NVR save area (BPF_PPC_STACK_SAVE). Whereas, the
> offset of tail_call_cnt in the trampoline frame is after the
> stack alignment padding. BPF JIT logic could become complex
> when dealing with frame-sensitive offset calculation of
> tail_call_cnt. Having the same offset in both frames is the
> desired objective.
> 
> The trampoline frame does not have a BPF_PPC_STACK_SAVE area.
> Introducing it leads to under-utilization of extra memory meant
> only for the offset alignment of tail_call_cnt.
> Another challenge is the variable alignment padding sitting at
> the bottom of the trampoline frame, which requires additional
> handling to compute tail_call_cnt offset.
> 
> This patch addresses the above issues by moving tail_call_cnt
> to the bottom of the stack frame at offset 0 for both types
> of frames. This saves additional bytes required by BPF_PPC_STACK_SAVE
> in trampoline frame, and a common offset computation for
> tail_call_cnt serves both frames.
> 
> The changes in this patch are required by the third patch in the
> series, where the 'reference to tail_call_info' of the main frame
> is copied into the trampoline frame from the previous frame.
> 
> Signed-off-by: Abhishek Dubey <adubey@...ux.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit.h        |  4 ++++
>   arch/powerpc/net/bpf_jit_comp64.c | 31 ++++++++++++++++++++-----------
>   2 files changed, 24 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 8334cd667bba..45d419c0ee73 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -72,6 +72,10 @@
>   	} } while (0)
>   
>   #ifdef CONFIG_PPC64
> +
> +/* for tailcall counter */
> +#define BPF_PPC_TAILCALL        8

This needs to be defined outside of CONFIG_PPC64 ifdef because from 
patch 3 it is used in bpf_jit_comp.c which is also built on powerpc32.

> +
>   /* If dummy pass (!image), account for maximum possible instructions */
>   #define PPC_LI64(d, i)		do {					      \
>   	if (!image)							      \