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Re: [Qemu-devel] [libvirt] [PATCH 6/7] qemu: Probe QEMU binary for host

From: Eric Blake
Subject: Re: [Qemu-devel] [libvirt] [PATCH 6/7] qemu: Probe QEMU binary for host CPU
Date: Tue, 23 Jul 2013 10:27:05 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7

[adding qemu]

On 07/23/2013 10:19 AM, Daniel P. Berrange wrote:
> On Tue, Jul 23, 2013 at 06:11:35PM +0200, Jiri Denemark wrote:
>> Since QEMU and kvm may filter some host CPU features or add efficiently
>> emulated features, asking QEMU binary for host CPU data provides
>> better results when we later use the data for building guest CPUs.
>> ---

>> +        virCommandAddArg(cmd, "-machine");
>> +        virCommandAddArgFormat(cmd, "%s,accel=kvm",
>> +                               qemuCaps->machineTypes[0]);
>> +

> This code is causing us to invoke the QEMU binary multiple times,
> which is something we worked really hard to get away from. I really,
> really don't like this as an approach. QEMU needs to be able to
> give us the data we need here without multiple invocations.
> eg, by allowing the monitor command to specify 'kvm' vs 'qemu'
> when asking for data, so you can interrogate it without having
> to re-launch it with different accel=XXX args.

We don't know if -machine accel=kvm is a valid option until after
issuing some QMP commands, but now we are forced to reinvoke a new qemu
with -machine accel=kvm enabled in order to get the query to give us
accurate answers.  I agree that this is less than desirable; hopefully
the qemu folks can help us figure out a solution, now that we are
bringing attention to the issue.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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