Re: [qemu-s390x] [PATCHv3 2/4] qmp: add query-cpus-fast

[Eric Blake]

On 02/15/2018 08:40 AM, Viktor Mihajlovski wrote:
> On 15.02.2018 15:19, Eric Blake wrote:
> > On 02/15/2018 04:20 AM, Viktor Mihajlovski wrote:
> > > From: Luiz Capitulino <address@hidden>
> > >
> > > The query-cpus command has an extremely serious side effect:
> > > it always interrupts all running vCPUs so that they can run
> > > ioctl calls. This can cause a huge performance degradation for
> > > some workloads. And most of the information retrieved by the
> > > ioctl calls are not even used by query-cpus.
> > >
> > > This commit introduces a replacement for query-cpus called

> > You know, we have no back-compat guarantees on HMP.  We could make 'info
> > cpu' just ALWAYS call query-cpus-fast, with no HMP counterpart for the
> > slower query-cpus, and without needing a deprecation period.  But I'll
> > leave that up to David if that makes more sense.
> Ditching info cpus_fast would make me happy as well, because it would
> cause less headache on the libvirt side of things.

Why is libvirt using HMP in the first place?  Libvirt should always be 
using the QMP command, when one exists.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org