Re: [RFC] kvm: x86: export vCPU halted state to sysfs

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Feb 01, 2018 at 12:54:41PM -0500, Luiz Capitulino wrote:
> 
> Libvirt needs to know when a vCPU is halted. To get this information,
> libvirt has started using the query-cpus command from QEMU. However,
> if in kernel irqchip is in use, query-cpus will force all vCPUs
> to user-space since they have to issue the KVM_GET_MP_STATE ioctl.
> This has catastrophic implications to low-latency workloads like
> KVM-RT and zero packet loss with DPDK. To make matters worse, there's
> an OpenStack service called ceilometer that causes libvirt to
> issue query-cpus every few minutes.
> 
> The solution proposed in this patch is to export the vCPU
> halted state in the already existing vcpu directory in sysfs.
> This way, libvirt can read the vCPU halted state from sysfs and avoid
> using the query-cpus command. This solution seems to be sufficient
> for libvirt needs, but it has the following cons:
> 
>  * vcpu information in sysfs lives in a debug directory, so
>    libvirt would be basing its API on debug info

Is this part of regular sysfs mount point, or does it require a
debug fs to be mounted separately at /sys/fs/debug ?

>  * Currently, only x86 supports the vcpu dir in sysfs, so
>    we'd have to expand this to other archs (should be doable)

Yep, that would be fairly important


Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|