On 2016/09/26 11:00AM, Daniel Borkmann wrote: > 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....@linux.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?
Yes, this set is contained within arch/powerpc, so Michael can take this through his tree. The other set with updates to samples/bpf can probably go through David's tree. - Naveen
