Re: MPIDR Aff0 question

[Andrew Jones <drjones@redhat.com>]

On Thu, Feb 04, 2016 at 06:51:06PM +0000, Marc Zyngier wrote:
> Hi Drew,
> 
> On 04/02/16 18:38, Andrew Jones wrote:
> > 
> > Hi Marc and Andre,
> > 
> > I completely understand why reset_mpidr() limits Aff0 to 16, thanks
> > to Andre's nice comment about ICC_SGIxR. Now, here's my question;
> > it seems that the Cortex-A{53,57,72} manuals want to further limit
> > Aff0 to 4, going so far as to say bits 7:2 are RES0. I'm looking
> > at userspace dictating the MPIDR for KVM. QEMU tries to model the
> > A57 right now, so to be true to the manual, Aff0 should only address
> > four PEs, but that would generate a higher trap cost for SGI broadcasts
> > when using KVM. Sigh... what to do?
> 
> There are two things to consider:
> 
> - The GICv3 architecture is perfectly happy to address 16 CPUs at Aff0.
> - ARM cores are designed to be grouped in clusters of at most 4, but
> other implementations may have very different layouts.
> 
> If you want to model something matches reality, then you have to follow
> what Cortex-A cores do, assuming you are exposing Cortex-A cores. But
> absolutely nothing forces you to (after all, we're not exposing the
> intricacies of L2 caches, which is the actual reason why we have
> clusters of 4 cores).

Thanks Marc. I'll take the question of whether or not deviation, in
the interest of optimal gicv3 use, is OK to QEMU.

> 
> > Additionally I'm looking at adding support to represent more complex
> > topologies in the guest MPIDR (sockets/cores/threads). I see Linux
> > currently expects Aff2:socket, Aff1:core, Aff0:thread when threads
> > are in use, and Aff1:socket, Aff0:core, when they're not. Assuming
> > there are never more than 4 threads to a core makes the first
> > expectation fine, but the second one would easily blow the 2 Aff0
> > bits alloted, and maybe even a 4 Aff0 bit allotment.
> > 
> > So my current thinking is that always using Aff2:socket, Aff1:cluster,
> > Aff0:core (no threads allowed) would be nice for KVM, and allowing up
> > to 16 cores to be addressed in Aff0. As it seems there's no standard
> > for MPIDR, then that could be the KVM guest "standard".
> > 
> > TCG note: I suppose threads could be allowed there, using
> > Aff2:socket, Aff1:core, Aff0:thread (no more than 4 threads)
> 
> I'm not sure why you'd want to map a given topology to a guest (other
> than to give the illusion of a particular system). The affinity register
> does not define any of this (as you noticed). And what would Aff3 be in
> your design? Shelve? Rack? ;-)

:-) Currently Aff3 would be unused, as there doesn't seem to be a need
for it, and as some processors don't have it, it would only complicate
things to use it sometimes.

> 
> What would the benefit of defining a "socket"?

That's a good lead in for my next question. While I don't believe
there needs to be any relationship between socket and numa node, I
suspect on real machines there is, and quite possibly socket == node.
Shannon is adding numa support to QEMU right now. Without special
configuration there's no gain other than illusion, but with pinning,
etc. the guest numa nodes will map to host nodes, and thus passing
that information on to the guest's kernel is useful. Populating a
socket/node affinity field seems to me like a needed step. But,
question time, is it? Maybe not. Also, the way Linux currently
handles non-thread using MPIDRs (Aff1:socket, Aff0:core) throws a
wrench at the Aff2:socket, Aff1:"cluster", Aff0:core(max 16) plan.
Either the plan or Linux would need to be changed.

Thanks,
drew
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