Messages in this thread | | | Date | Thu, 11 Nov 2021 15:24:59 +0100 | Subject | Re: [PATCH RFC] KVM: x86: Drop arbitraty KVM_SOFT_MAX_VCPUS | From | Paolo Bonzini <> |
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On 11/11/21 14:47, Vitaly Kuznetsov wrote: > KVM_CAP_NR_VCPUS is used to get the "recommended" maximum number of > VCPUs and arm64/mips/riscv report num_online_cpus(). Powerpc reports > either num_online_cpus() or num_present_cpus(), s390 has multiple > constants depending on hardware features. On x86, KVM reports an > arbitrary value of '710' which is supposed to be the maximum tested > value but it's possible to test all KVM_MAX_VCPUS even when there are > less physical CPUs available. > > Drop the arbitrary '710' value and return num_online_cpus() on x86 as > well. The recommendation will match other architectures and will mean > 'no CPU overcommit'. > > For reference, QEMU only queries KVM_CAP_NR_VCPUS to print a warning > when the requested vCPU number exceeds it. The static limit of '710' > is quite weird as smaller systems with just a few physical CPUs should > certainly "recommend" less. > > Suggested-by: Eduardo Habkost <ehabkost@redhat.com> > Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Yes, this is a good idea. We cannot move it entirely to common code due to POWER's handling of secondary threads in hypervisors; still, this is as close as we can get to a common idea of what KVM_CAP_NR_VCPUS means.
Queued, thanks.
Paolo
> --- > arch/x86/include/asm/kvm_host.h | 1 - > arch/x86/kvm/x86.c | 2 +- > 2 files changed, 1 insertion(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index 88fce6ab4bbd..0232a00598f2 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -38,7 +38,6 @@ > #define __KVM_HAVE_ARCH_VCPU_DEBUGFS > > #define KVM_MAX_VCPUS 1024 > -#define KVM_SOFT_MAX_VCPUS 710 > > /* > * In x86, the VCPU ID corresponds to the APIC ID, and APIC IDs > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index ac83d873d65b..91ef1b872b90 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -4137,7 +4137,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) > r = !static_call(kvm_x86_cpu_has_accelerated_tpr)(); > break; > case KVM_CAP_NR_VCPUS: > - r = KVM_SOFT_MAX_VCPUS; > + r = num_online_cpus(); > break; > case KVM_CAP_MAX_VCPUS: > r = KVM_MAX_VCPUS; >
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