On Fri, 2022-02-25 at 15:42 +0000, David Woodhouse wrote: > On Fri, 2022-02-25 at 17:11 +0200, Maxim Levitsky wrote: > > On Fri, 2022-02-25 at 14:56 +0000, David Woodhouse wrote: > > > On Fri, 2022-02-25 at 16:46 +0200, Maxim Levitsky wrote: > > > > Assuming that this is approved and accepted upstream, > > > > that is even better that my proposal of doing this > > > > when APICv is enabled. > > > > > > > > Since now apic id is always read only, now we should not > > > > forget to clean up some parts of kvm like kvm_recalculate_apic_map, > > > > which are not needed anymore > > > > > > Can we also now optimise kvm_get_vcpu_by_id() so it doesn't have to do > > > a linear search over all the vCPUs when there isn't a 1:1 > > > correspondence with the vCPU index? > > > > I don't think so since vcpu id can still be set by userspace to anything, > > and this is even used to encode topology in it. > > Yes, but it can only be set at vCPU creation time and it has to be > unique. > > > However a hash table can still be used there to speed it up regardless of > > read-only apic id IMHO. > > > > Or, even better than a hash table, I see that KVM already > > limits vcpu_id to KVM_MAX_VCPUS * 4 with a comment that only two extra > > bits of topology are used: > > We already have the kvm_apic_map which provides a fast lookup. The key > point here is that the APIC ID can't *change* from vcpu->vcpu_id any > more, so we can actually use the kvm_apic_map for kvm_get_vcpu_by_id() > now, can't we? > Right! I wrote my response partially when I still assumed that vcpu_id can be any 32 bit number (thus hash table), and later checked that it is capped by KVM_MAX_VCPUS * 4 which isn't a big number, plus as I now see in the kvm_recalculate_apic_map the map is dynamically allocated up to the max apic id. (technically speaking an array is a hash table). Now the map would only be needed to be rebuit few times when new vCPUs are added, and can be used to locate vcpu by its apic id. I so hope that this patch is accepted so all of this could be done. Best regards, Maxim Levitsky
