Re: [PATCH 2/3] bpf powerpc: implement support for tail calls

[Alexei Starovoitov]

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@xxxxxxxxxxxxxxxxxx>
> 
> 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.