Hi Rusty,
Rusty Russell <rusty@xxxxxxxxxxxxxxx> writes:
> Anthony Liguori <aliguori@xxxxxxxxxx> 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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