On 12/12/18 21:15, Nadav Amit wrote: >> On Dec 12, 2018, at 10:33 AM, Edward Cree <ec...@solarflare.com> wrote: >> >> AIUI the outline version uses a tail-call (i.e. jmpq *target) rather than an >> additional call and ret. So I wouldn't expect it to be too expensive. >> More to the point, it seems like it's easier to get right than the inline >> version, and if we get the inline version working later we can introduce it >> without any API change, much as Josh's existing patches have both versions >> behind a Kconfig switch. > I see. For my outlined blocks I used the opposite approach - a call followed > by jmp That's what Josh did too. I.e. caller calls the trampoline, which jmps to the callee; later it rets, taking it back to the caller. Perhaps I wasn't clear. The point is that there's still only one call and one ret.
>> I was working on the assumption that it would be opt-in, wrapping a macro >> around indirect calls that are known to have a fairly small number of hot >> targets. There are plenty of indirect calls in the kernel that are only >> called once in a blue moon, e.g. in control-plane operations like ethtool; >> we don't really need to bulk up .text with trampolines for all of them. > On the other hand, I’m not sure the static_call interface is so intuitive. > And extending it into “dynamic_call” might be even worse. As I initially > used an opt-in approach, I can tell you that it was very exhausting. Well, if it's done with a gcc plugin after all, then it wouldn't be too hard to make it opt-out. One advantage of the explicit opt-in dynamic_call, though, which can be seen in my patch is that multiple call sites can share the same learning-state, if they're expected to call the same set of functions. An opt-out approach would automatically give each indirect call statement its own individual BTB. Either way, I think the question is orthogonal to what the trampolines themselves look like (and even to the inline vs outline question). -Ed
