Re: [PATCH net-next 1/4] bpf: allow bpf programs to tail-call other bpf programs

From: Daniel Borkmann <daniel@iogearbox.net>
To: Alexei Starovoitov <ast@plumgrid.com>
Cc: "David S. Miller" <davem@davemloft.net>,
	Ingo Molnar <mingo@kernel.org>,
	Michael Holzheu <holzheu@linux.vnet.ibm.com>,
	Zi Shen Lim <zlim.lnx@gmail.com>,
	linux-api@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH net-next 1/4] bpf: allow bpf programs to tail-call other bpf programs
Date: Thu, 21 May 2015 18:17:07 +0200	[thread overview]
Message-ID: <555E0503.8010302@iogearbox.net> (raw)
In-Reply-To: <1432079946-9878-2-git-send-email-ast@plumgrid.com>

On 05/20/2015 01:59 AM, Alexei Starovoitov wrote:
> introduce bpf_tail_call(ctx, &jmp_table, index) helper function
> which can be used from BPF programs like:
> int bpf_prog(struct pt_regs *ctx)
> {
>    ...
>    bpf_tail_call(ctx, &jmp_table, index);
>    ...
> }
> that is roughly equivalent to:
> int bpf_prog(struct pt_regs *ctx)
> {
>    ...
>    if (jmp_table[index])
>      return (*jmp_table[index])(ctx);
>    ...
> }
> The important detail that it's not a normal call, but a tail call.
> The kernel stack is precious, so this helper reuses the current
> stack frame and jumps into another BPF program without adding
> extra call frame.
> It's trivially done in interpreter and a bit trickier in JITs.
> In case of x64 JIT the bigger part of generated assembler prologue
> is common for all programs, so it is simply skipped while jumping.
> Other JITs can do similar prologue-skipping optimization or
> do stack unwind before jumping into the next program.
>
...
> Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>

LGTM, thanks!

Acked-by: Daniel Borkmann <daniel@iogearbox.net>