Re: [Qemu-arm] MPIDR Aff0 question

[Mark Rutland]

On Fri, Feb 05, 2016 at 10:23:53AM +0100, Andrew Jones wrote:
> On Thu, Feb 04, 2016 at 06:51:06PM +0000, Marc Zyngier wrote:
 > 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. 

I don't think it's necessary.

When using ACPI, NUMA info comes from SRAT+SLIT, and the MPIDR.Aff*
fields do not provide NUMA topology info. I expect the same to be true
with DT using something like numa-distance-map [1].

> 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.

The topology can be explicitly overridden in DT using cpu-map [2]. I
don't know what the story for ACPI is.

Mark.

[1] 
http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/404057.html
[2] 
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/arm/topology.txt?h=v4.5-rc2&id=36f90b0a2ddd60823fe193a85e60ff1906c2a9b3