From: Alexei Starovoitov <ast@plumgrid.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "David S. Miller" <davem@davemloft.net>,
Ingo Molnar <mingo@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Michael Holzheu <holzheu@linux.vnet.ibm.com>,
Zi Shen Lim <zlim.lnx@gmail.com>,
Linux API <linux-api@vger.kernel.org>,
Network Development <netdev@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper
Date: Tue, 19 May 2015 17:14:33 -0700 [thread overview]
Message-ID: <555BD1E9.5000000@plumgrid.com> (raw)
In-Reply-To: <CALCETrVZyB+m8Oaad1JGwrLx1rTUoL0F6Afrvg_hQWiYyTTang@mail.gmail.com>
On 5/19/15 5:11 PM, Andy Lutomirski wrote:
> On Tue, May 19, 2015 at 4:59 PM, Alexei Starovoitov <ast@plumgrid.com> wrote:
>> bpf_tail_call() arguments:
>> ctx - context pointer
>> jmp_table - one of BPF_MAP_TYPE_PROG_ARRAY maps used as the jump table
>> index - index in the jump table
>>
>> In this implementation x64 JIT bypasses stack unwind and jumps into the
>> callee program after prologue, so the callee program reuses the same stack.
>>
>> The logic can be roughly expressed in C like:
>>
>> u32 tail_call_cnt;
>>
>> void *jumptable[2] = { &&label1, &&label2 };
>>
>> int bpf_prog1(void *ctx)
>> {
>> label1:
>> ...
>> }
>>
>> int bpf_prog2(void *ctx)
>> {
>> label2:
>> ...
>> }
>>
>> int bpf_prog1(void *ctx)
>> {
>> ...
>> if (tail_call_cnt++ < MAX_TAIL_CALL_CNT)
>> goto *jumptable[index]; ... and pass my 'ctx' to callee ...
>>
>> ... fall through if no entry in jumptable ...
>> }
>>
>
> What causes the stack pointer to be right? Is there some reason that
> the stack pointer is the same no matter where you are in the generated
> code?
that's why I said 'it's _roughly_ expressed in C' this way.
Stack pointer doesn't change. It uses the same stack frame.
next prev parent reply other threads:[~2015-05-20 0:14 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-19 23:59 [PATCH net-next 0/4] bpf: introduce bpf_tail_call() helper Alexei Starovoitov
2015-05-19 23:59 ` [PATCH net-next 1/4] bpf: allow bpf programs to tail-call other bpf programs Alexei Starovoitov
2015-05-20 0:13 ` Andy Lutomirski
2015-05-20 0:18 ` Alexei Starovoitov
2015-05-21 16:20 ` Andy Lutomirski
2015-05-21 16:40 ` Alexei Starovoitov
2015-05-21 16:43 ` Andy Lutomirski
2015-05-21 16:53 ` Alexei Starovoitov
2015-05-21 16:57 ` Andy Lutomirski
2015-05-21 17:16 ` Alexei Starovoitov
2015-05-21 16:17 ` Daniel Borkmann
2015-05-19 23:59 ` [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper Alexei Starovoitov
2015-05-20 0:11 ` Andy Lutomirski
2015-05-20 0:14 ` Alexei Starovoitov [this message]
2015-05-20 16:05 ` Andy Lutomirski
2015-05-20 16:29 ` Alexei Starovoitov
2015-05-19 23:59 ` [PATCH net-next 3/4] samples/bpf: bpf_tail_call example for tracing Alexei Starovoitov
2015-05-19 23:59 ` [PATCH net-next 4/4] samples/bpf: bpf_tail_call example for networking Alexei Starovoitov
2015-05-21 21:08 ` [PATCH net-next 0/4] bpf: introduce bpf_tail_call() helper David Miller
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