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

[Juergen Gross <jgross@xxxxxxxx>]

On 03.03.22 09:33, Juergen Gross wrote:
> 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.
BTW, in theory this approach could be generalized for other function
vectors in the hypervisor, too (maybe even all?).


Juergen

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature
Description: OpenPGP digital signature