Re: [Qemu-devel] [libvirt] [PATCH for-4.0 v4 0/2] virtio: Provide version-specific variants of virtio PCI devices

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: "Ján Tomko" <jtomko@redhat.com>
Cc: "Peter Krempa" <pkrempa@redhat.com>,
	"Kevin Wolf" <kwolf@redhat.com>,
	"Eduardo Habkost" <ehabkost@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	libvir-list@redhat.com, "Jason Wang" <jasowang@redhat.com>,
	"Cornelia Huck" <cohuck@redhat.com>,
	"Stefan Hajnoczi" <stefanha@redhat.com>,
	"Andrea Bolognani" <abologna@redhat.com>,
	"Amit Shah" <amit@kernel.org>,
	qemu-devel@nongnu.org, Max@angien.pipo.sk,
	"Cleber Rosa" <crosa@redhat.com>,
	Gonglei <arei.gonglei@huawei.com>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	"Laine Stump" <laine@redhat.com>,
	"Caio Carrara" <ccarrara@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>, Reitz <mreitz@redhat.com>,
	"Philippe Mathieu-Daudé" <philmd@redhat.com>,
	"Wainer dos Santos Moschetta" <wainersm@redhat.com>
Subject: Re: [Qemu-devel] [libvirt] [PATCH for-4.0 v4 0/2] virtio: Provide version-specific variants of virtio PCI devices
Date: Wed, 6 Mar 2019 09:10:08 +0000	[thread overview]
Message-ID: <20190306091008.GC20806@redhat.com> (raw)
In-Reply-To: <20190306083032.GZ1252@lpt>

On Wed, Mar 06, 2019 at 09:30:32AM +0100, Ján Tomko wrote:
> On Wed, Mar 06, 2019 at 08:41:48AM +0100, Peter Krempa wrote:
> > On Tue, Mar 05, 2019 at 16:56:43 +0100, Andrea Bolognani wrote:
> > > On Tue, 2019-03-05 at 15:38 +0100, Gerd Hoffmann wrote:
> > 
> > [...]
> > 
> > > So I agree neither scenario is exactly perfect, but I still think
> > > adding non-transitional alias devices would overall be more
> > > user-friendly.
> > 
> > I don't think it makes sense to add it at the qemu level. From libvirt's
> > point of view users should be shielded from any qemu impl detail or
> > inconsistency as libvirt is the 'user friendly'[1] layer. In qemu the
> > devices would be the same and thus does not make sense to do that
> > because it would be more confusing.
> > 
> > You can argue that we should add the alias at the libvirt level though.
> > 
> 
> You can, but please don't.

Indeed, at the libvirt level we've always tried to take the view that
there should be one way to expressing each concept/feature. Adding
new names / xml elements that duplicate existing supported concepts
to make things "consistent" is a slippery slope becasue there are
100's of places to which that can apply when you have hindsight.

It is not going to make a significant difference to how "user friendly"
libvirt is - that is not a core goal in its own right at the API / XML
schema level. It is can be a factor in deciding between multiple competing
designs when first adding a feature, but it isn't a reason to add duplication
in the API / XML.

Regards,
Daniel
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