Re: [PATCH v2 2/5] KVM: x86: hyperv: introduce vp_index_to_vcpu_idx mapping

[Roman Kagan]

On Fri, Jun 29, 2018 at 12:26:23PM +0200, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:
> 
> > On Thu, Jun 28, 2018 at 03:53:10PM +0200, Vitaly Kuznetsov wrote:
> >> While it is easy to get VP index from vCPU index the reverse task is hard.
> >> Basically, to solve it we have to walk all vCPUs checking if their VP index
> >> matches. For hypercalls like HvFlushVirtualAddress{List,Space}* and the
> >> upcoming HvSendSyntheticClusterIpi* where a single CPU may be specified in
> >> the whole set this is obviously sub-optimal.
> >> 
> >> As VP index can be set to anything <= U32_MAX by userspace using plain
> >> [0..MAX_VP_INDEX] array is not a viable option. Use condensed sorted
> >> array with logarithmic search complexity instead. Use RCU to make read
> >> access as fast as possible and maintain atomicity of updates.
> >
> > Quoting TLFS 5.0C section 7.8.1:
> >
> >> Virtual processors are identified by using an index (VP index). The
> >> maximum number of virtual processors per partition supported by the
> >> current implementation of the hypervisor can be obtained through CPUID
> >> leaf 0x40000005. A virtual processor index must be less than the
> >> maximum number of virtual processors per partition.
> >
> > so this is a dense index, and VP_INDEX >= KVM_MAX_VCPUS is invalid.  I
> > think we're better off enforcing this in kvm_hv_set_msr and keep the
> > translation simple.  If the algorithm in get_vcpu_by_vpidx is not good
> > enough (and yes it can be made to return NULL early on vpidx >=
> > KVM_MAX_VCPUS instead of taking the slow path) then a simple index array
> > of KVM_MAX_VCPUS entries should certainly do.
> 
> Sure, we can use pre-allocated [0..KVM_MAX_VCPUS] array instead and put
> limits on what userspace can assign VP_INDEX to. Howver, while thinking
> about it I decided to go with the more complex condensed array approach
> because the tendency is for KVM_MAX_VCPUS to grow and we will be
> pre-allocating more and more memory for no particular reason (so I think
> even 'struct kvm_vcpu *vcpus[KVM_MAX_VCPUS]' in 'struct kvm' will need
> to be converted to something else eventually). 

We're talking of kilobytes here.  I guess this is going to be the least
of the scalability problems.

> Anyway, I'm flexible and if you think we should go this way now I'll do
> this in v3. We can re-think this when we later decide to raise
> KVM_MAX_VCPUS significantly.

Although there's no strict requirement for that I think every sensible
userspace will allocate VP_INDEX linearly resulting in it being equal to
KVM's vcpu index.  So we've yet to see a case where get_vcpu_by_vpidx
doesn't take the fast path.  If it ever starts appearing in the profiles
we may consider optimiziing it but ATM I don't even think introducing
the translation array is justified.

Roman.