Re: [libvirt] [Qemu-devel] [PATCH] kvm: warn if num cpus is greater than num recommended

On 22/08/2013, at 12:39, Andrew Jones <drjones(a)redhat.com&gt; wrote: > The comment in kvm_max_vcpus() states that it's using the recommended > procedure from the kernel API documentation to get the max number > of vcpus that kvm supports. It is, but by always returning the > maximum number supported. The maximum number should only be used > for development purposes. qemu should check KVM_CAP_NR_VCPUS for > the recommended number of vcpus. This patch adds a warning if a user > specifies a number of cpus between the recommended and max. > > Signed-off-by: Andrew Jones <drjones(a)redhat.com&gt; CCing libvir-list. It is probably interesting for libvirt to expose or warn about the recommended VCPU limit somehow, and in this case a simple warning on stderr won't be enough. > --- > kvm-all.c | 45 +++++++++++++++++++++++++++------------------ > 1 file changed, 27 insertions(+), 18 deletions(-) > > diff --git a/kvm-all.c b/kvm-all.c > index 716860f617455..9092e13ae60ea 100644 > --- a/kvm-all.c > +++ b/kvm-all.c > @@ -1313,24 +1313,24 @@ static int kvm_irqchip_create(KVMState *s) > return 0; > } > > -static int kvm_max_vcpus(KVMState *s) > +/* Find number of supported CPUs using the recommended > + * procedure from the kernel API documentation to cope with > + * older kernels that may be missing capabilities. > + */ > +static int kvm_recommended_vcpus(KVMState *s) > { > int ret; > > - /* Find number of supported CPUs using the recommended > - * procedure from the kernel API documentation to cope with > - * older kernels that may be missing capabilities. > - */ > - ret = kvm_check_extension(s, KVM_CAP_MAX_VCPUS); > - if (ret) { > - return ret; > - } > ret = kvm_check_extension(s, KVM_CAP_NR_VCPUS); > - if (ret) { > - return ret; > - } > + return (ret) ? ret : 4; > +} > > - return 4; > +static int kvm_max_vcpus(KVMState *s) > +{ > + int ret; > + > + ret = kvm_check_extension(s, KVM_CAP_MAX_VCPUS); > + return (ret) ? ret : kvm_recommended_vcpus(s); > } > > int kvm_init(void) > @@ -1383,12 +1383,21 @@ int kvm_init(void) > goto err; > } > > - max_vcpus = kvm_max_vcpus(s); > + max_vcpus = kvm_recommended_vcpus(s); > if (smp_cpus > max_vcpus) { > - ret = -EINVAL; > - fprintf(stderr, "Number of SMP cpus requested (%d) exceeds max cpus " > - "supported by KVM (%d)\n", smp_cpus, max_vcpus); > - goto err; > + fprintf(stderr, > + "Warning: Number of SMP cpus requested (%d) exceeds " > + "recommended cpus supported by KVM (%d)\n", > + smp_cpus, max_vcpus); > + > + max_vcpus = kvm_max_vcpus(s); > + if (smp_cpus > max_vcpus) { > + ret = -EINVAL; > + fprintf(stderr, "Number of SMP cpus requested (%d) exceeds " > + "max cpus supported by KVM (%d)\n", > + smp_cpus, max_vcpus); > + goto err; > + }

> } > > s->vmfd = kvm_ioctl(s, KVM_CREATE_VM, 0);