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Date: Mon, 26 Sep 2016 11:00:36 +0200 From: Daniel Borkmann <daniel@...earbox.net> To: "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>, Alexei Starovoitov <alexei.starovoitov@...il.com> CC: Ananth N Mavinakayanahalli <ananth@...ibm.com>, Alexei Starovoitov <ast@...com>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, "David S. Miller" <davem@...emloft.net> Subject: Re: [PATCH 2/3] bpf powerpc: implement support for tail calls On 09/26/2016 10:56 AM, Naveen N. Rao wrote: > On 2016/09/24 03:30AM, Alexei Starovoitov wrote: >> On Sat, Sep 24, 2016 at 12:33:54AM +0200, Daniel Borkmann wrote: >>> 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 <naveen.n.rao@...ux.vnet.ibm.com> >>> >>> 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! >> >> Great stuff. In this circumstances approach 2 makes sense to me as well. > > Alexie, Daniel, > Thanks for the quick review! The patches would go via Michael's tree (same way as with the JIT itself in the past), right?
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