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Re: [Qemu-devel] [PATCH 0/2] i386: "unavailable-features" QOM property

From: Jiri Denemark
Subject: Re: [Qemu-devel] [PATCH 0/2] i386: "unavailable-features" QOM property
Date: Fri, 10 May 2019 13:33:03 +0200
User-agent: Mutt/1.11.4 (2019-03-13)

On Thu, May 09, 2019 at 13:08:25 -0300, Eduardo Habkost wrote:
> On Thu, May 09, 2019 at 05:26:16PM +0200, Jiri Denemark wrote:
> > On Thu, May 09, 2019 at 10:56:17 -0300, Eduardo Habkost wrote:
> > > On Thu, May 09, 2019 at 03:35:37PM +0200, Jiri Denemark wrote:
> > > > Would this unavailable-features property contain only canonical names of
> > > > the features or all possible aliases of all features? For example,
> > > > "tsc-adjust" can also be spelled as "tsc_adjust". When calling
> > > > query-cpu-model-expansion, we have a way to request all variants by
> > > > running full expansion on the result of a previous static expansion. Can
> > > > we get something like this for unavailable-features too?
> > > 
> > > I'd like to avoid that, and refer only to the canonical names.
> > > 
> > > Could you explain the use case you have in mind, so we can look
> > > for alternatives?
> > 
> > Libvirt only knows about a single spelling for each CPU feature and
> > quite often it is not the canonical variant. Thus libvirt would only
> > recognize features for which the name known by libvirt is the canonical
> > name.
> > 
> > We could theoretically make the translation in libvirt, but it's not
> > going to be future proof. If a new spelling is introduced, it's because
> > the old one is not considered correct and the new one becomes the
> > canonical version. When libvirt doesn't have the translation (libvirt is
> > older or just didn't catch up yet) we have a big problem.
> > 
> > I guess a good alternative would be some way of querying all CPU feature
> > names and their aliases. If I'm not mistaken, we can either get a list
> > of canonical names or all names, but without any clue which names
> > actually refer to a single feature.
> Right.  But (as described below) if you want to know the status a
> specific feature you already know about (even if you are using
> the old spelling), qom-get should work for you.

Yeah, since we'll be checking all features anyway, we can detect enabled
and disabled features at the same time. However, we don't know whether a
specific feature was disabled because we did not ask for it to be
enabled (remember, CPU model definition may differ between libvirt and
QEMU) or because it was filtered out.

Depending on the domain XML used to start a domain, libvirt may not (and
usually will not) refuse to start a domain for which QEMU filtered out
some CPU features. Of course, once the domain is running, the checking
becomes very strict and libvirt would refuse to start the domain on
another host during migration if any feature is filtered out.

Thus libvirt stores all features QEMU filtered out when a domain was
started in the non-strict way. So we need to match the features in the
unavailable-features list with our features. Just checking for the list
being empty is not sufficient.


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