On Thu, Jun 30, 2022, Vitaly Kuznetsov wrote: > Kyle Meyer <kyle.meyer@xxxxxxx> writes: > > > Increase KVM_MAX_VCPUS to 4096 when the maximum number of supported CPUs > > (NR_CPUS) is greater than 1024. Changelog says "greater than 1024", code says "at least 1024". > > The Hyper-V TLFS doesn't allow more than 64 sparse banks, which allows a > > maximum of 4096 virtual CPUs. Limit KVM's maximum number of virtual CPUs > > to 4096 to avoid exceeding that limit. > > In theory, it's just TLB flush and IPI hypercalls which have this > limitation. Strictly speaking, guest can have more than 4096 vCPUs, > it'll just have to do IPIs/TLB flush in a different way. Yeah, I don't see any reason to arbitrarily limit this to 4096. And conversely, I don't see any reason to force it to 4096 if NR_CPUS < 4096, it seems highly unlikely that there's a use case for oversubscribing vCPUs in a single VM when there are more than 1024 pCPUs.
