linux-kernel - Re: [PATCH v2 0/4] Static calls

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Date:   Wed, 12 Dec 2018 21:36:22 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Nadav Amit <namit@...are.com>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH v2 0/4] Static calls

On 12/12/18 21:15, Nadav Amit wrote:
>> On Dec 12, 2018, at 10:33 AM, Edward Cree <ecree@...arflare.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