Re: [PATCH 2/2] kvm/arm: Add mp_affinity for arm vcpu

[Marc Zyngier <maz@kernel.org>]

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...