From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760740AbcIWWd7 (ORCPT ); Fri, 23 Sep 2016 18:33:59 -0400 Received: from www62.your-server.de ([213.133.104.62]:54973 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755042AbcIWWd6 (ORCPT ); Fri, 23 Sep 2016 18:33:58 -0400 Message-ID: <57E5ADD2.7090308@iogearbox.net> Date: Sat, 24 Sep 2016 00:33:54 +0200 From: Daniel Borkmann User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: "Naveen N. Rao" , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, Michael Ellerman CC: Alexei Starovoitov , "David S. Miller" , Ananth N Mavinakayanahalli Subject: Re: [PATCH 2/3] bpf powerpc: implement support for tail calls References: <40b65ab2bb3a48837ab047a70887de3ccd70c56b.1474661927.git.naveen.n.rao@linux.vnet.ibm.com> In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Authenticated-Sender: daniel@iogearbox.net Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/23/2016 10:35 PM, Naveen N. Rao wrote: > Tail calls allow JIT'ed eBPF programs to call into other JIT'ed eBPF > programs. This can be achieved either by: > (1) retaining the stack setup by the first eBPF program and having all > subsequent eBPF programs re-using it, or, > (2) by unwinding/tearing down the stack and having each eBPF program > deal with its own stack as it sees fit. > > To ensure that this does not create loops, there is a limit to how many > tail calls can be done (currently 32). This requires the JIT'ed code to > maintain a count of the number of tail calls done so far. > > Approach (1) is simple, but requires every eBPF program to have (almost) > the same prologue/epilogue, regardless of whether they need it. This is > inefficient for small eBPF programs which may not sometimes need a > prologue at all. As such, to minimize impact of tail call > implementation, we use approach (2) here which needs each eBPF program > in the chain to use its own prologue/epilogue. This is not ideal when > many tail calls are involved and when all the eBPF programs in the chain > have similar prologue/epilogue. However, the impact is restricted to > programs that do tail calls. Individual eBPF programs are not affected. > > We maintain the tail call count in a fixed location on the stack and > updated tail call count values are passed in through this. The very > first eBPF program in a chain sets this up to 0 (the first 2 > instructions). Subsequent tail calls skip the first two eBPF JIT > instructions to maintain the count. For programs that don't do tail > calls themselves, the first two instructions are NOPs. > > Signed-off-by: Naveen N. Rao Thanks for adding support, Naveen, that's really great! I think 2) seems fine as well in this context as prologue size can vary quite a bit here, and depending on program types likelihood of tail call usage as well (but I wouldn't expect deep nesting). Thanks a lot!