Re: [Qemu-devel] redirecting stdio from a VM guest

[Eric Blake]

On 05/28/2014 06:51 AM, Alexander Binun wrote:
> Dear Friends, 
>    First of all, great thanks for your support! I would ask yet one question.
> 
> I have a programs running on a VM guest. Its output is valuable (for VM 
> introspection) so I want to let the host module know about it. I prefer to 
> redirect  ' stdio" of a guest into a device at the host (the latter has the 
> name "mydev" and the driver is written for it). 
> 
> The command string for qemu will look like qemu -serial /dev/mydev ... But I 
> would like to modify the libvirt definition XML file (in /etc/libvirt/qemu). 
> Stefan notes in his blog 
> (http://blog.vmsplice.net/2011/04/how-to-pass-qemu-command-line-options.html) 
> that the options will look like:
> 
> <qemu:commandline>
>    <qemu:arg value='-serial'/>
>    <qemu:arg value='/dev/mydev>
> </qemu:commandline>

Use of this syntax moves you into the realm of unsupported behavior. It
should only be a last resort for debugging or development purposes, with
a request made to the libvirt people to turn your usage into a formal
feature.

> 
> But the link referenced from the log mentioned above 
> (http://wiki.libvirt.org/page/QEMUSwitchToLibvirt) provides a different XML 
> mapping for the "serial" command:
> 
> <serial type='pty'>
>      <target port='0'/>
>    </serial>

Yes, the <serial> block is the formal feature that you should be using,
rather than resorting to <qemu:commandline>.

> 
> How should I map my preference -serial /dev/mydev into the XML ? 

You're better off asking libvirt usage questions on the libvirt list
rather than here on the qemu list (address@hidden).  That
said, http://libvirt.org/formatdomain.html#elementsCharFle makes it
sound like you want
 <serial type="file">
  <source path="/dev/mydev"/>
  <target port="0"/>
 </serial>

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

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