On 2020-09-17 09:04, Andrew Jones wrote:
On Thu, Sep 17, 2020 at 08:47:42AM +0100, Marc Zyngier wrote:
On 2020-09-17 03:30, Ying Fang wrote:
> Allow userspace to set MPIDR using vcpu ioctl KVM_ARM_SET_MP_AFFINITY,
> so that we can support cpu topology for arm.
MPIDR has *nothing* to do with CPU topology in the ARM architecture.
I encourage you to have a look at the ARM ARM and find out how often
the word "topology" is used in conjunction with the MPIDR_EL1
register.
Hi Marc,
I mostly agree. However, the CPU topology descriptions use MPIDR to
identify PEs. If userspace wants to build topology descriptions then
it either needs to
1) build them after instantiating all KVM VCPUs in order to query KVM
for each MPIDR, or
2) have a way to ask KVM for an MPIDR of given VCPU ID in advance
(maybe just a scratch VCPU), or
3) have control over the MPIDRs so it can choose them when it likes,
use them for topology descriptions, and then instantiate KVM VCPUs
with them.
I think (3) is the most robust approach, and it has the least overhead.
I don't disagree with the goal, and not even with the choice of
implementation (though I have huge reservations about its quality).
But the key word here is *userspace*. Only userspace has a notion of
how MPIDR values map to the assumed topology. That's not something
that KVM does nor should interpret (aside from the GIC-induced Aff0
brain-damage). So talking of "topology" in a KVM kernel patch sends
the wrong message, and that's all this remark was about.
M.
--
Jazz is not dead. It just smells funny...
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