On Wed, Sep 21, 2016 at 04:26:31PM +0100, Daniel P. Berrange wrote:
On Wed, Sep 21, 2016 at 05:10:09PM +0200, Martin Kletzander wrote:
On Wed, Sep 21, 2016 at 04:01:23PM +0100, Daniel P. Berrange wrote:
> I'm not a fan of the idea of silently picking a different device
> for the guest behind the applications back. By not exposing the
> different device types with a "model" attribute, we miss a way
> to report to the application which models are supported by the
> QEMU they're using - eg via domain capabilities.
>
> This in turn means the application doesn't know whether they're
> getting the new or old version, and so don't know if they're going
> to have working migration or not.
>
> If we expanded the XML to include model, then application can
> explicitly request the new model (which supports migration) and
> know that they'll get a startup failure if not supported, as
> opposed to silently falling back to the non-migratable version.
>
> Also, it makes life hard for us if the ivshmem-plain device wants
> to support use of the 'server' attribute in the future, as we will
> then not know which to create.
>
> We've often been burnt in the past by trying todo magic like this,
> instead of explicitly representing stuff in the XML, so I think we
> should be being explicit about ivshmem models here.
>
> Of course, if we do add <model> support, we have to allow for it
> to be missing for sake of upgrades. So there's a question of which
> model we should select as the default, if not seen in the XML.
>

If selecting the newest one whenever the element is missing is fine,
then I'm OK with that.  But that would change the device when upgrading
libvirt (without user intervention), which you didn't like IIUC.

Yes, I don't think we can do that - at least note exactly in the way
you do it in this patch. eg Looking at the ivshmem code in QEMU there
is this comment about guest interupt setup:

    * Do nothing unless the device actually uses INTx.  Here's how
    * the device variants signal interrupts, what they put in PCI
    * config space:
    * Device variant    Interrupt  Interrupt Pin  MSI-X cap.
    * ivshmem-plain         none            0         no
    * ivshmem-doorbell     MSI-X            1        yes(1)
    * ivshmem,msi=off       INTx            1         no
    * ivshmem,msi=on       MSI-X            1(2)     yes(1)
    * (1) if guest enabled MSI-X
    * (2) the device lies

Note that neither ivshmem-plain or ivshmem-doorbell support use of
INTx for interupts. I'm also concerned about the footnote (2) there,
as that seems to imply that ivshmem,msi=on, is not in fact the
same as ivshmem-doorbell, because ivshmem lies about the interrupt
pin (whatever that means).

I'm inclined so that that we should do

 if (ivshmem exists) {
    use ivshmem
 } else {
    if (server) {
       if(msi) {
           use ivshmem-doorbell
        } else {
           error config unsupported
        }
    } else {
       use ivshmem-plain
    }
 }

That way if a distro compiles-out 'ivshmem' we'll use one of the
new devices if they're available, otherwise we'll stick with the
conversative behaviour of using the legacy device for guaranteed
bug compatibility.


OK, we can do that.  But before I go and do this variant, I would like
to clarify two more things (so that I can hope that's the last variant I
have to post) =)  Should we support setting the role to 'peer'/'master'
(with ivshmem that maps to role=peer/master and with ivshmem-plain that
maps to master=on/off)?  Basically master means that the domain can
migrate (with the data being copied) and peer (or master=off) has
migration disabled in qemu, so we would disable that as well.  That's
why hotplug is being implemented, basically.  Currently we don't use
that parameter, which means role=auto.  That is special value that ends
up being 'master' for non-server, for server it tries to pick correctly.
Shouldn't be used, but rather explicitly stated (or just peer for
everyone and copy the data yourself).  New ivshmem defaults to master=off.

Regards,
Daniel
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