Re: [Xen-devel] [libvirt] domXML modeling question

[Daniel P. Berrangé <berrange@xxxxxxxxxx>]

On Mon, Mar 04, 2019 at 04:42:32PM -0700, Jim Fehlig wrote:
> Adding xen-devel to cc in case anyone there wants to comment on my latest
> proposal...
> 
> On 2/20/19 5:20 PM, Jim Fehlig wrote:
> > There have been a few requests [1][2] to support Xen's max_grant_frames
> > setting in libvirt domXML, but I'm not quite sure how to model it. The
> > documentation [3] on this setting states:
> > 
> > Specify the maximum number of grant frames the domain is allowed to have.  
> > This
> > value controls how many pages the domain is able to grant access to for 
> > other
> > domains, needed e.g. for the operation of paravirtualized devices.  The 
> > default
> > is settable via xl.conf(5).
> 
> I've sent a patch to introduce an analogous default in the libvirt libxl 
> driver
> 
> https://www.redhat.com/archives/libvir-list/2019-March/msg00123.html
> 
> > 
> > It smells of a <memtune> setting, e.g. the amount of memory a domain can
> > share, but doesn't map to any of the existing settings. A new subelement
> > <shared_limit> doesn't feel right. Does anyone suggest a better way of
> > modeling max_grant_frames?
> 
> After discussing the max_grant_frames setting a bit more with Juergen I had
> the idea to model it as IO buffer space (or DMA space) of a xenbus
> "controller". All PV devices in the guest connect to the xenbus controller
> and make use of the available I/O buffer space. Guests with more PV devices
> requiring more buffer can increase the space on the xenbus controller
> device.
> 
> One small wrinkle in this idea is that we currently don't model xenbus in
> libvirt. I'd need to add support for a new xenbus controller type and start
> implicitly creating it when creating guests with PV devices, similar to
> auto-creation of controllers in the qemu driver. Also, there is no existing
> controller setting for specifying buffer space. Perhaps a 'ram' attribute
> could be added, similar to specifying memory for <video> devices? E.g.
> 
>   <controller type='xenbus' ram='256'/>
> 
> Any opinion on this approach? Or other ideas for modeling this setting
> in libvirt?

Regardless of max grant frames support I think modeling xenbus as a
<controller> is a reasonable thing to want to do. I don't have a
preference on whether you call it "ram" or explicitly "maxGrantFrames"
as an attribute.


Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel