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Re: [libvirt] [PATCH 16/18] qemu: Support scsi controller model=virtio-{non-}transitional



On Tue, 2019-01-22 at 16:27 -0500, Cole Robinson wrote:
> On 01/22/2019 12:39 PM, Cole Robinson wrote:
> > On 01/21/2019 11:49 AM, Andrea Bolognani wrote:
> > > Completely agree with the rationale here; however, in order to
> > > really match the way other devices are handled, I think we should
> > > make it so you can use
> > > 
> > >   <controller type='scsi' model='virtio'/>
> > > 
> > > as well, which of course would behave the same as the currently
> > > available version. What do you think?
> > 
> > Yes I agree it would be nice to add that option. I suggest we make it a
> > separate issue from this series though incase it's a contentious idea,
> > v2 is already shaping up to be ~30 patches...

I don't think that's going to be particularly controversial, and we
should really make sure we get all the user-facing bits in at the
same time IMHO, so I'd say go for it... If you're really unsure
about it you can add that model in a separate patch right after this
one, that way if someone happens not to like that we can drop it and
otherwise we can squash them together before pushing.

> > > Using strstr() here is kinda crude, especially since the caller has
> > > enough information to pass the appropriate virDomainControllerType
> > > value, both in this case and later on for serial controllers.
> > > 
> > > I'd say just add yet another argument to the function. Yeah, it
> > > starts to get quite unsightly, but I'd really rather not resort to
> > > string comparison when a nice, type safe enum will do.
> > 
> > Yeah it's hacky. Adding another arg here is going to add extra pain if
> > when this is merged into qemuBuildVirtioStr, most callers are going have
> > NULL arguments, and an increased chance of new future callers passing in
> > incorrect values and accidentally triggering a wrong code path. I feel
> > like this is another argument for the separated BuildTransitional or
> > whatever, but I'm not sold either way so I'll stick with your suggestion
> 
> What do you think of this approach? See the attached two patches. It
> adds a domain_conf.c function virDomainDeviceSetData which makes it
> easier to create a virDomainDeviceDef. qemuBuildVirtioDevStr callers now
> pass in a virDomainDeviceType and the specific DefPtr for their device
> (virDomainDiskDefPtr, virDomainNetDefPtr etc). qemuBuildVirtioDevStr
> then turns that into a virDomainDeviceDef and acts on that locally.
> 
> Saves having to extend the argument list several times (like this
> example above, and the net->model string example). Seperately I think
> virDomainDeviceSetData can be used to clean up some device interactions
> elsewhere too

I like it! I actually wanted to suggest something like that earlier,
but for some reason I thought it would be more complicated than it
turned out to be...

Better yet, you don't even need to add that switch statement to
qemuBuildVirtioDevStr(): you can just use virDomainDeviceGetInfo()
instead, thus making the code shorter and nicer :)

-- 
Andrea Bolognani / Red Hat / Virtualization


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