Re: [PATCH RFC] xen/sched: Optimise when only one scheduler is compiled in

[Juergen Gross <jgross@xxxxxxxx>]

On 03.03.22 01:40, Andrew Cooper wrote:
> When only one scheduler is compiled in, function pointers can be optimised to
> direct calls, and the hooks hardened against controlflow hijacking.
>
> RFC for several reasons.
>
> 1) There's an almost beautiful way of not introducing MAYBE_SCHED() and hiding
>     the magic in REGISTER_SCHEDULER(), except it falls over
>     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91765 which has no comment or
>     resolution at all.
>
> 2) A different alternative which almost works is to remove the indirection in
>     .data.schedulers, but the singleton scheduler object can't be both there
>     and in .init.rodata.cf_clobber.
>
> 3) I can't think of a way of build time check to enforce that new schedulers
>     get added to the preprocessor magic.
>
> And the blocker:
> 4) This isn't compatible with how sched_idle_ops get used for granularity > 1.
>
> Suggestions very welcome.
Did you consider to generate the needed code dynamically instead?

I guess this could even be extended to avoid function pointers
completely using the same technique as in my hypercall series.

In order to avoid the need for a central table the per-scheduler
hooks could use standard names (as most of them do already).

I think I could come up with a patch in a few hours if you like
that approach.


Juergen

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