Re: [PATCH] KVM: x86: Increase KVM_MAX_VCPUS to 4096

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <seanjc@google.com> 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@hpe.com>
>> ---
>> 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@redhat.com.
>
> 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