netdev - Re: [PATCH bpf-next 0/4] bpf: tailcall: Eliminate max_entries and bpf

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Message-ID: <CAADnVQLxo1uPbutGNKrv=f=bSVkzxOfSof0ea8n7VvqsaU+S3w@mail.gmail.com>
Date: Wed, 14 Jan 2026 08:04:38 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Leon Hwang <leon.hwang@...ux.dev>, bpf <bpf@...r.kernel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Puranjay Mohan <puranjay@...nel.org>, Xu Kuohai <xukuohai@...weicloud.com>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	"David S . Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>, 
	"H . Peter Anvin" <hpa@...or.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, kernel-patches-bot@...com
Subject: Re: [PATCH bpf-next 0/4] bpf: tailcall: Eliminate max_entries and
 bpf_func access at runtime

On Wed, Jan 14, 2026 at 3:28 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Fri, Jan 02, 2026 at 04:10:01PM -0800, Alexei Starovoitov wrote:
> > On Fri, Jan 2, 2026 at 7:01 AM Leon Hwang <leon.hwang@...ux.dev> wrote:
> > >
> > > This patch series optimizes BPF tail calls on x86_64 and arm64 by
> > > eliminating runtime memory accesses for max_entries and 'prog->bpf_func'
> > > when the prog array map is known at verification time.
> > >
> > > Currently, every tail call requires:
> > >   1. Loading max_entries from the prog array map
> > >   2. Dereferencing 'prog->bpf_func' to get the target address
> > >
> > > This series introduces a mechanism to precompute and cache the tail call
> > > target addresses (bpf_func + prologue_offset) in the prog array itself:
> > >   array->ptrs[max_entries + index] = prog->bpf_func + prologue_offset
> > >
> > > When a program is added to or removed from the prog array, the cached
> > > target is atomically updated via xchg().
> > >
> > > The verifier now encodes additional information in the tail call
> > > instruction's imm field:
> > >   - bits 0-7:   map index in used_maps[]
> > >   - bits 8-15:  dynamic array flag (1 if map pointer is poisoned)
> > >   - bits 16-31: poke table index + 1 for direct tail calls
> > >
> > > For static tail calls (map known at verification time):
> > >   - max_entries is embedded as an immediate in the comparison instruction
> > >   - The cached target from array->ptrs[max_entries + index] is used
> > >     directly, avoiding the 'prog->bpf_func' dereference
> > >
> > > For dynamic tail calls (map pointer poisoned):
> > >   - Fall back to runtime lookup of max_entries and prog->bpf_func
> > >
> > > This reduces cache misses and improves tail call performance for the
> > > common case where the prog array is statically known.
> >
> > Sorry, I don't like this. tail_calls are complex enough and
> > I'd rather let them be as-is and deprecate their usage altogether
> > instead of trying to optimize them in certain conditions.
> > We have indirect jumps now. The next step is indirect calls.
> > When it lands there will be no need to use tail_calls.
> > Consider tail_calls to be legacy. No reason to improve them.
>
> hi,
> I'd like to make tail calls available in sleepable programs. I still
> need to check if there's technical reason we don't have that, but seeing
> this answer I wonder you'd be against that anyway ?

tail_calls are not allowed in sleepable progs?
I don't remember such a limitation.
What prevents it?
prog_type needs to match, so all sleepable progs should be fine.
The mix and match is problematic due to rcu vs srcu life times.

> fyi I briefly discussed that with Andrii indicating that it might not
> be worth the effort at this stage.

depending on complexity of course.