Re: [PATCH v3 3/5] KVM: x86: hyperv: use get_vcpu_by_vpidx() in kvm_hv_flush_tlb()

[Vitaly Kuznetsov]

Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:

> On Fri, Jun 29, 2018 at 04:14:53PM +0200, Vitaly Kuznetsov wrote:
>> VP_INDEX almost always matches VCPU id and get_vcpu_by_vpidx() is fast,
>> use it instead of traversing full vCPU list every time.
>> 
>> To support the change switch kvm_make_vcpus_request_mask() to checking
>> vcpu_id instead of vcpu index,
>
> I'm afraid you can't do this: vcpu_id (== apic id) can be sparse, i.e.
> it's not very well suited for bitmaps and can exceed the max number of
> vcpus.

True. The bitmap should be of KVM_MAX_VCPU_ID size, not
KVM_MAX_VCPUS.

Unfortunately there's no convenient way to get VCPU idx from VCPU
id, kvm_vcpu_get_idx() just walks the whole list :-( I see two possible
options:
1) Add vcpu_idx fields to struct kvm_vcpu
2) Keep the change expecting masks of KVM_MAX_VCPU_ID in
kvm_make_vcpus_request_mask(). KVM_MAX_VCPU_ID is currently 1023 so our
bitmaps will be 16 longs long. Not sure if it's too much.

-- 
  Vitaly