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. > arch/x86/include/asm/kvm_host.h | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index 3bc146dfd38d..91a01fa17fa7 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -39,7 +39,11 @@ > > #define __KVM_HAVE_ARCH_VCPU_DEBUGFS > > +#ifdef CONFIG_MAXSMP > +#define KVM_MAX_VCPUS 4096 > +#else > #define KVM_MAX_VCPUS 1024 > +#endif Rather than tightly couple this to MAXSMP, what if we add a Kconfig? I know of at least one scenario, SVM's AVIC/x2AVIC, where it would be desirable to configure KVM to a much smaller maximum. The biggest downside I can think of is that KVM selftests would need to be updated (they assume the max is >=512), and some of the tests might be completely invalid if KVM_MAX_VCPUS is too low (<256?). E.g. diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 60d430b4650f..8704748e35d9 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -39,7 +39,7 @@ #define __KVM_HAVE_ARCH_VCPU_DEBUGFS -#define KVM_MAX_VCPUS 1024 +#define KVM_MAX_VCPUS CONFIG_KVM_MAX_NR_VCPUS /* * In x86, the VCPU ID corresponds to the APIC ID, and APIC IDs diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig index ed90f148140d..b0f92eb77f78 100644 --- a/arch/x86/kvm/Kconfig +++ b/arch/x86/kvm/Kconfig @@ -151,6 +151,17 @@ config KVM_PROVE_MMU If in doubt, say "N". +config KVM_MAX_NR_VCPUS + int "Maximum vCPUs per VM" + default "4096" if MAXSMP + default "1024" + range 1 4096 + depends on KVM + help + Set the maximum number of vCPUs for a single VM. Larger values + increase the memory footprint of each VM regardless of how many vCPUs + are actually created (though the memory increase is relatively small). + config KVM_EXTERNAL_WRITE_TRACKING bool
