Re: [RFC PATCH 3/4] KVM: stats: Add ioctl commands to pull statistics in binary format

Hi Paolo,

On Wed, Mar 10, 2021 at 8:55 AM Paolo Bonzini pbonzini@redhat.com wrote:

On 10/03/21 01:30, Jing Zhang wrote:

...

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 383df23514b9..87dd62516c8b 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -3464,6 +3464,51 @@ static long kvm_vcpu_ioctl(struct file *filp, r = kvm_arch_vcpu_ioctl_set_fpu(vcpu, fpu); break; }

• case KVM_STATS_GET_INFO: {
  

•         struct kvm_stats_info stats_info;
  

•         r = -EFAULT;
  

•         stats_info.num_stats = VCPU_STAT_COUNT;
  

•         if (copy_to_user(argp, &stats_info, sizeof(stats_info)))
  

•                 goto out;
  

•         r = 0;
  

•         break;
  

• }
  

• case KVM_STATS_GET_NAMES: {
  

•         struct kvm_stats_names stats_names;
  

•         r = -EFAULT;
  

•         if (copy_from_user(&stats_names, argp, sizeof(stats_names)))
  

•                 goto out;
  

•         r = -EINVAL;
  

•         if (stats_names.size < VCPU_STAT_COUNT * KVM_STATS_NAME_LEN)
  

•                 goto out;
  

•         r = -EFAULT;
  

•         if (copy_to_user(argp + sizeof(stats_names),
  

•                         kvm_vcpu_stat_strings,
  

•                         VCPU_STAT_COUNT * KVM_STATS_NAME_LEN))
  

The only reason to separate the strings in patch 1 is to pass them here. But this is a poor API because it imposes a limit on the length of the statistics, and makes that length part of the binary interface.

I would prefer a completely different interface, where you have a file descriptor that can be created and associated to a vCPU or VM (or even to /dev/kvm). Having a file descriptor is important because the fd can

We are considering about how to create the file descriptor. It might be risky to create an extra fd for every vCPU. It will easily hit the fd limit for the process or the system for machines running a ton of small VMs. Looks like creating an extra file descriptor for every VM is a better option. And then we can check per vCPU stats through Ioctl of this VM fd by passing the vCPU index. What do you think?

be passed to a less-privileged process that takes care of gathering the metrics

The result of reading the file descriptor could be either ASCII or binary. IMO the real cost lies in opening and reading a multitude of files rather than in the ASCII<->binary conversion.

The format could be one of the following:

• binary:

4 bytes flags (always zero) 4 bytes number of statistics 4 bytes offset of the first stat description 4 bytes offset of the first stat value stat descriptions:

• 4 bytes for the type (for now always zero: uint64_t)
• 4 bytes for the flags (for now always zero)
• length of name
• name

statistics in 64-bit format

• text:

stat1_name uint64 123 stat2_name uint64 456 ...

What do you think?

Paolo