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.
