Re: [PATCH] x86/smp: Factor out parts of native_smp_prepare_cpus()

[Boris Ostrovsky]

On 11/8/21 10:11 AM, Peter Zijlstra wrote:
> On Tue, Nov 02, 2021 at 07:36:36PM -0400, Boris Ostrovsky wrote:
> > Commit 66558b730f25 ("sched: Add cluster scheduler level for x86")
> > introduced cpu_l2c_shared_map mask which is expected to be initialized
> > by smp_op.smp_prepare_cpus(). That commit only updated
> > native_smp_prepare_cpus() version but not xen_pv_smp_prepare_cpus().
> > As result Xen PV guests crash in set_cpu_sibling_map().
> >
> > While the new mask can be allocated in xen_pv_smp_prepare_cpus() one can
> > see that both versions of smp_prepare_cpus ops share a number of common
> > operations that can be factored out. So do that instead.
> >
> > Fixes: 66558b730f25 ("sched: Add cluster scheduler level for x86")
> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Thanks! I'll go stick that somewhere /urgent (I've had another report on
> that here:
>
>    https://lkml.kernel.org/r/20211105074139.GE174703@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> )


Thank you. (I don't see this message btw)


> But looking at those functions; there seems to be more spurious
> differences. For example, the whole sched_topology thing.


I did look at that and thought this should be benign given that Xen PV is not really topology-aware. I didn't see anything that would be a cause for concern but perhaps you can point me to things I missed.


> Should we re-architect this whole smp_prepare_cpus() thing instead? Have
> a common function and a guest function? HyperV for instance seems to
> call native_smp_prepare_cpus() and then does something extra (as does
> xen_hvm).


Something like


void smp_prepare_cpus()

{

    // Code that this patch moved to smp_prepare_cpus_common();


   smp_ops.smp_prepare_cpus();  // Including baremetal

}


?


XenHVM and hyperV will need to call native smp_op too. Not sure this will be prettier than what it is now?



-boris


-boris