Re: [Qemu-devel] [PATCH v2] kvm: warn if num cpus is greater than num recommended

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Aug 23, 2013 at 03:24:37PM +0200, Andrew Jones 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.
> 
> v2:
> Incorporate tests for max_cpus, which specifies the maximum number
> of hotpluggable cpus. An additional note is that the message for
> the fail case was slightly changed, 'exceeds max cpus' to
> 'exceeds the maximum cpus'. If this is unacceptable change for
> users like libvirt, then I'll need to spin a v3.
> 
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
>  kvm-all.c | 69 ++++++++++++++++++++++++++++++++++++---------------------------
>  1 file changed, 40 insertions(+), 29 deletions(-)
> 
> diff --git a/kvm-all.c b/kvm-all.c
> index a2d49786365e3..021f5f47e53da 100644
> --- a/kvm-all.c
> +++ b/kvm-all.c
> @@ -1322,24 +1322,20 @@ 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;
> -    }
> +    int ret = kvm_check_extension(s, KVM_CAP_NR_VCPUS);
> +    return (ret) ? ret : 4;
> +}
>  
> -    return 4;
> +static int kvm_max_vcpus(KVMState *s)
> +{
> +    int ret = kvm_check_extension(s, KVM_CAP_MAX_VCPUS);
> +    return (ret) ? ret : kvm_recommended_vcpus(s);
>  }
>  
>  int kvm_init(void)
> @@ -1347,11 +1343,19 @@ int kvm_init(void)
>      static const char upgrade_note[] =
>          "Please upgrade to at least kernel 2.6.29 or recent kvm-kmod\n"
>          "(see http://sourceforge.net/projects/kvm).\n";
> +    struct {
> +        const char *name;
> +        int num;
> +    } num_cpus[] = {
> +        { "SMP",          smp_cpus },
> +        { "hotpluggable", max_cpus },
> +        { NULL, }
> +    }, *nc = num_cpus;
> +    int soft_vcpus_limit, hard_vcpus_limit;
>      KVMState *s;
>      const KVMCapabilityInfo *missing_cap;
>      int ret;
>      int i;
> -    int max_vcpus;
>  
>      s = g_malloc0(sizeof(KVMState));
>  
> @@ -1392,19 +1396,26 @@ int kvm_init(void)
>          goto err;
>      }
>  
> -    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;
> -    }
> +    /* check the vcpu limits */
> +    soft_vcpus_limit = kvm_recommended_vcpus(s);
> +    hard_vcpus_limit = kvm_max_vcpus(s);
>  
> -    if (max_cpus > max_vcpus) {
> -        ret = -EINVAL;
> -        fprintf(stderr, "Number of hotpluggable cpus requested (%d) exceeds max cpus "
> -                "supported by KVM (%d)\n", max_cpus, max_vcpus);
> -        goto err;
> +    while (nc->name) {
> +        if (nc->num > soft_vcpus_limit) {
> +            fprintf(stderr,
> +                    "Warning: Number of %s cpus requested (%d) exceeds "
> +                    "the recommended cpus supported by KVM (%d)\n",
> +                    nc->name, nc->num, soft_vcpus_limit);
> +
> +            if (nc->num > hard_vcpus_limit) {
> +                ret = -EINVAL;
> +                fprintf(stderr, "Number of %s cpus requested (%d) exceeds "
> +                        "the maximum cpus supported by KVM (%d)\n",
> +                        nc->name, nc->num, hard_vcpus_limit);
> +                goto err;
> +            }
> +        }
> +        nc++;
>      }
>  
>      s->vmfd = kvm_ioctl(s, KVM_CREATE_VM, 0);
> -- 
> 1.8.1.4

ACK