From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eduardo Habkost <ehabkost@redhat.com>
Subject: Re: [Qemu-devel] [RFC] kvm: x86: export vCPU halted state to sysfs
Date: Fri, 2 Feb 2018 15:38:46 -0200
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Cc: Luiz Capitulino <lcapitulino@redhat.com>,
        Peter Krempa <pkrempa@redhat.com>, kvm@vger.kernel.org,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        libvir-list@redhat.com, Markus Armbruster <armbru@redhat.com>,
        qemu-devel@nongnu.org,
        Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        John Ferlan <jferlan@redhat.com>, pbonzini@redhat.com
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
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On Fri, Feb 02, 2018 at 05:23:59PM +0000, Dr. David Alan Gilbert wrote:
> * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > (CCing qemu-devel)
> > 
> > On Fri, Feb 02, 2018 at 09:21:59AM -0500, Luiz Capitulino wrote:
> > > On Fri, 2 Feb 2018 14:19:38 +0000
> > > Daniel P. Berrangé <berrange@redhat.com> wrote:
> > > > On Fri, Feb 02, 2018 at 12:15:54PM -0200, Eduardo Habkost wrote:
> > [...]
> > > > > It would be also interesting to update QEMU QMP documentation to
> > > > > clarify the arch-specific semantics of "halted".  
> > > > 
> > > > Any also especially clarify the awful performance implications of running
> > > > this particular query command. In general I would not expect query-xxx
> > > > monitor commands to interrupt all vcpus, so we should clearly warn about
> > > > this !
> > > 
> > > Or deprecate it...
> > 
> > We could deprecate the expensive fields on query-cpus, and move
> > them to a more expensive query-cpu-state command.  I believe most
> > users of query-cpus are only interested in qom_path, thread_id,
> > and topology info.
> 
> Would that data be available without the bql?  I ask because if it is
> then a small advantage to having a separate command is that the command
> could be marked OOB with Peter's new series and never take the lock.

We would need a mechanism to safely walk the CPU list without the
BQL, so I wouldn't bother trying to make a OOB-capable version of
query-cpus unless really necessary.

-- 
Eduardo