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

[Jing Zhang <jingzhangos@google.com>]

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.
Agreed. I am considering returning the length of stats name strings in
the kvm_stats_info structure instead of exporting it as a constant in uapi,
which would put no limit on the length of the stats name strings.
>
> 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
> be passed to a less-privileged process that takes care of gathering the
> metrics
Separate file descriptor solution is very tempting. We are still considering it
seriously. Our biggest concern is that the metrics gathering/handling process
is not necessary running on the same node as the one file descriptor belongs to.
It scales better to pass metrics data directly than to pass file descriptors.
>
> 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.
Agreed.
>
> 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?
The binary format presented above is very flexible. I understand why it is
organized this way.
In our situation, the metrics data could be pulled periodically as short as
half second. They are used by different kinds of monitors/triggers/alerts.
To enhance efficiency and reduce traffic caused by metrics passing, we
treat all metrics info/data as two kinds. One is immutable information,
which doesn't change in a given system boot. The other is mutable
data (statistics data), which is pulled/transferred periodically at a high
frequency.

>
> Paolo
>

Thanks,
Jing