| Message ID | 20211116163443.88707-7-vkuznets@redhat.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | KVM: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS and re-purpose it on x86 | expand |
On 11/16/21 17:34, 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> Given that KVM_SOFT_MAX_VCPUS has already been dropped in 5.16, I changed the commit message to KVM: x86: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS It doesn't make sense to return the recommended maximum number of vCPUs which exceeds the maximum possible number of vCPUs. Paolo
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..18a00a7c23bc 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 = min_t(unsigned int, num_online_cpus(), KVM_MAX_VCPUS); break; case KVM_CAP_MAX_VCPUS: r = KVM_MAX_VCPUS;
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> --- arch/x86/include/asm/kvm_host.h | 1 - arch/x86/kvm/x86.c | 2 +- 2 files changed, 1 insertion(+), 2 deletions(-)