Re: [Qemu-devel] [PATCHv5 0/5] add query-cpu-fast and related s390 changes

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 16 Feb 2018 17:08:36 +0100
Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com> wrote:

> v5 synopsis: Split out HMP changes from Patch 2 into Patch 5. Please
>    	     re-review, as I've removed the a-b/r-b from Patch 2
> 	     as well.
> 
> This series consolidates patches around a performance issue
> caused by the usage of QMP query-cpus.
> 
> A performance issue was found in an OpenStack environment, where
> ceilometer was collecting domain statistics with libvirt. The domain
> statistics reported by libvirt include the vCPU halted state, which 
> in turn is retrieved with QMP query-cpus.
> 
> This causes two issues:
> 1. Performance: on most architectures query-cpus needs to issue a KVM ioctl
>    to find out whether a vCPU was halted. This is not the case for s390
>    but query-cpus is always causing the vCPU to exit the VM.
> 
> 2. Semantics: on x86 and other architectures, halted is a highly transient
>    state, which is likely to have already changed shortly after the state
>    information has been retrieved. This is not the case for s390, where
>    halted is an indication that the vCPU is stopped, meaning its not
>    available to the guest operating system until it has been restarted.

Thanks, queued to s390-next.