Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR

[Anthony Liguori <aliguori@us.ibm.com>]

Hi Rusty,

Rusty Russell <rusty@rustcorp.com.au> writes:

> Anthony Liguori <aliguori@us.ibm.com> writes:
>> 4) Do virtio-pcie, make it PCI-e friendly (drop the IO BAR completely), give
>>    it a new device/vendor ID.   Continue to use virtio-pci for existing
>>    devices potentially adding virtio-{net,blk,...}-pcie variants for
>>    people that care to use them.
>
> Now you have a different compatibility problem; how do you know the
> guest supports the new virtio-pcie net?

We don't care.

We would still use virtio-pci for existing devices.  Only new devices
would use virtio-pcie.

> If you put a virtio-pci card behind a PCI-e bridge today, it's not
> compliant, but AFAICT it will Just Work.  (Modulo the 16-dev limit).

I believe you can put it in legacy mode and then there isn't the 16-dev
limit.  I believe the only advantage of putting it in native mode is
that then you can do native hotplug (as opposed to ACPI hotplug).

So sticking with virtio-pci seems reasonable to me.

> I've been assuming we'd avoid a "flag day" change; that devices would
> look like existing virtio-pci with capabilities indicating the new
> config layout.

I don't think that's feasible.  Maybe 5 or 10 years from now, we switch
the default adapter to virtio-pcie.

>> I think 4 is the best path forward.  It's better for users (guests
>> continue to work as they always have).  There's less confusion about
>> enabling PCI-e support--you must ask for the virtio-pcie variant and you
>> must have a virtio-pcie driver.  It's easy to explain.
>
> Removing both forward and backward compatibility is easy to explain, but
> I think it'll be harder to deploy.  This is your area though, so perhaps
> I'm wrong.

My concern is that it's not real backwards compatibility.

>> It also maps to what regular hardware does.  I highly doubt that there
>> are any real PCI cards that made the shift from PCI to PCI-e without
>> bumping at least a revision ID.
>
> Noone expected the new cards to Just Work with old OSes: a new machine
> meant a new OS and new drivers.  Hardware vendors like that.

Yup.

> Since virtualization often involves legacy, our priorities might be
> different.

So realistically, I think if we introduce virtio-pcie with a different
vendor ID, it will be adopted fairly quickly.  The drivers will show up
in distros quickly and get backported.

New devices can be limited to supporting virtio-pcie and we'll certainly
provide a way to use old devices with virtio-pcie too.  But for
practical reasons, I think we have to continue using virtio-pci by
default.

Regards,

Anthony Liguori

>
> Cheers,
> Rusty.
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