Sean Christopherson <seanjc@xxxxxxxxxx> writes: > On Tue, Aug 15, 2023, Kyle Meyer wrote: >> Increase KVM_MAX_VCPUS to 4096 when MAXSMP is enabled. >> >> Notable changes (when MAXSMP is enabled): >> >> * KMV_MAX_VCPUS will increase from 1024 to 4096. >> * KVM_MAX_VCPU_IDS will increase from 4096 to 16384. >> * KVM_HV_MAX_SPARSE_VCPU_SET_BITS will increase from 16 to 64. >> * CPUID[HYPERV_CPUID_IMPLEMENT_LIMITS (0x40000005)].EAX will now be 4096. >> >> * struct kvm will increase from 39408 B to 39792 B. >> * struct kvm_ioapic will increase from 5240 B to 19064 B. >> >> * The following (on-stack) bitmaps will increase from 128 B to 512 B: >> * dest_vcpu_bitmap in kvm_irq_delivery_to_apic. >> * vcpu_mask in kvm_hv_flush_tlb. >> * vcpu_bitmap in ioapic_write_indirect. >> * vp_bitmap in sparse_set_to_vcpu_mask. >> >> Signed-off-by: Kyle Meyer <kyle.meyer@xxxxxxx> >> --- >> Virtual machines with 4096 virtual CPUs have been created on 32 socket >> Cascade Lake and Sapphire Rapids systems. >> >> 4096 is the current maximum value because of the Hyper-V TLFS. See >> BUILD_BUG_ON in arch/x86/kvm/hyperv.c, commit 79661c3, and Vitaly's >> comment on https://lore.kernel.org/all/87r136shcc.fsf@xxxxxxxxxx. > > Mostly out of curiosity, do you care about Hyper-V support? If not, at some > point it'd probably be worth exploring a CONFIG_KVM_HYPERV option to allow > disabling KVM's Hyper-V support at compile time so that we're not bound by the > restrictions of the TLFS. > (sorry for necroposting) While adding CONFIG_KVM_HYPERV to disable all-things-Hyper-V may make sense for some deployments (and as we already have CONFIG_KVM_XEN), I don't think we should forbid KVM_MAX_VCPUS > 4096 when it is enabled: 'general purpose' (distro) kernels are used both for hosting large Linux guests and Windows guests. Instead, I'd suggest we define KVM_MAX_HV_VCPUS as MIN(KVM_MAX_VCPUS, 4096) and then e.g. fail KVM_SET_CPUID[,2] if we already have > 4096 vCPUs + fail kvm_arch_vcpu_create() if we already have something-hyperv enabled on the already created vCPUs. -- Vitaly
