Re: [PATCH RFC 6/7] ARM64: KVM: Support heterogeneous system

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Mar 15, 2017 at 03:06:33PM +0100, Andrew Jones wrote:
> On Wed, Mar 15, 2017 at 02:36:45PM +0100, Christoffer Dall wrote:
> > > If QEMU wants to know
> > > whether or not the host it's running on is heterogeneous, then
> > > it can just query sysfs, rather than ask KVM.
> > > 
> > 
> > Can it?  Is this information available in a reliable way from userspace?
> 
> I don't know much (anything) about it, but, afaict, yes. See
> https://lkml.org/lkml/2017/1/19/380

The "capacity" of a CPU does *not* tell you if your system is
hetereogeneous. Two vastly different CPU implementations can stumble
upon the same capacity, and two identical implementations could be
assigned close but not identical capacities.

The "capacity" is purely a scheduler heuristic, and should not be relied
upon for functional correctness.

We have a sysfs interface to see the MIDR and REVIDR of (online) CPUs,
which can tell you. See Documentation/arm64/cpu-feature-registers.txt.

Whether a system is heterogeneous can change at runtime, as CPUs can be
brought online very late (e.g. if booted with maxcpus capped, or if we
get "real" hotplug in future).

Thanks,
Mark.