RE: [Xen-devel] Re: [PATCH] SMP dom0 boot fix

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> Thanks Kier. With this fix applied, I am able to boot SMP 
> dom0. The box I am testing on is x86_64 machine with two 
> hardware threads. However, if I turn on SMT in the Linux 
> configuration, the kernel takes a fault in early startup (a 
> NULL pointer reference at find_busiest_group +144). It 
> appears that the sched domain hierarchy is not correctly set up  here.
> Looking at the new smpboot.c, is turning on SMT support no 
> longer valid?

I don't think we want particularly want to expose CPU sibling
relationships to guests, at least not ones that don't have all their
vcpu's pinned. I think a better approach would probably just to have the
guest scheduler favour low numbered VCPUs and leave Xen to do the maping
to logical CPUs. This would get us most of the benefit of SMT
scheduling, the only trick we'd be missing is the reduced migration
penalty for moving a process between logical CPUs sharing an L2 cache.
If we felt this was a big deal, we'd need some way of reflecting the
current relationship via shared info or such like.  Eitherway, no good
can come of enabling SMT in the guest right now, though we should fix
the crash.

Ian 

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