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

[Daniel Borkmann <daniel@iogearbox.net>]

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@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?