Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

[Ard Biesheuvel]

On 9 November 2018 at 20:03, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Fri, Nov 09, 2018 at 06:53:21PM +0100, Ard Biesheuvel wrote:
>> > It's a bit fiddly since inline and out-of-line both use
>> > arch_static_call_transform(), but what I need to do is basically:
>> >
>> > - for out-of-line, the trampoline needs to be patched into a
>> > movn/movk/movk/br sequence if the target is too far
>> > - for inline, the trampoline itself needs to be patched from
>> > adrp/ldr/br (which does a load and a indirect branch) to
>> > movn/movk/movk/br (which uses immediates), and the call sites need to
>> > be patched into calls to the veneer if the target is out of range.
>> >
>> > So arch_static_call_transform() needs to know where the trampoline is
>> > for this use case, so could we perhaps add a 'void *orig' in the key
>> > struct that keeps track of the original value of 'addr' ?
>>
>> ... and pass it to arch_static_call_transform
>
> Ok.  So in both cases, you need both the call site address and the tramp
> address, right?  We could add a 'tramp' pointer to the key struct -- I
> assume that's what you mean?
>

Yes.

> And also change
>
>   void arch_static_call_transform(unsigned long insn, void *dest)
>
> to
>
>   void arch_static_call_transform(void *call_site, void *tramp, void *dest)
>

Indeed.