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

Gleb Natapov
2013-09-03T08:56:08Z

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>
Applied, thanks.

> ---
> 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

--
Gleb.