"Michael S. Tsirkin" <m...@redhat.com> writes:
> On Wed, Jun 05, 2013 at 10:46:15AM -0500, Anthony Liguori wrote:
>> Look, it's very simple.
> We only need to do it if we do a change that breaks guests.
>
> Please find a guest that is broken by the patches. You won't find any.
I think the problem in this whole discussion is that we're talking past
each other.
Here is my understanding:
1) PCI-e says that you must be able to disable IO bars and still have a
functioning device.
2) It says (1) because you must size IO bars to 4096 which means that
practically speaking, once you enable a dozen or so PIO bars, you run
out of PIO space (16 * 4k == 64k and not all that space can be used).
virtio-pci uses a IO bars exclusively today. Existing guest drivers
assume that there is an IO bar that contains the virtio-pci registers.
So let's consider the following scenarios:
QEMU of today:
1) qemu -drive file=ubuntu-13.04.img,if=virtio
This works today. Does adding an MMIO bar at BAR1 break this?
Certainly not if the device is behind a PCI bus...
But are we going to put devices behind a PCI-e bus by default? Are we
going to ask the user to choose whether devices are put behind a legacy
bus or the express bus?
What happens if we put the device behind a PCI-e bus by default? Well,
it can still work. That is, until we do something like this:
2) qemu -drive file=ubuntu-13.04.img,if=virtio -device virtio-rng
-device virtio-balloon..
Such that we have more than a dozen or so devices. This works
perfectly fine today. It works fine because we've designed virtio to
make sure it works fine. Quoting the spec:
"Configuration space is generally used for rarely-changing or
initialization-time parameters. But it is a limited resource, so it
might be better to use a virtqueue to update configuration information
(the network device does this for filtering, otherwise the table in the
config space could potentially be very large)."
In fact, we can have 100s of PCI devices today without running out of IO
space because we're so careful about this.
So if we switch to using PCI-e by default *and* we keep virtio-pci
without modifying the device IDs, then very frequently we are going to
break existing guests because the drivers they already have no longer
work.
A few virtio-serial channels, a few block devices, a couple of network
adapters, the balloon and RNG driver, and we hit the IO space limit
pretty damn quickly so this is not a contrived scenario at all. I would
expect that we frequently run into this if we don't address this problem.
So we have a few options:
1) Punt all of this complexity to libvirt et al and watch people make
the wrong decisions about when to use PCI-e. This will become yet
another example of KVM being too hard to configure.
2) Enable PCI-e by default and just force people to upgrade their
drivers.
3) Don't use PCI-e by default but still add BAR1 to virtio-pci
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.
I think 1 == 2 == 3 and I view 2 as an ABI breaker. libvirt does like
policy so they're going to make a simple decision and always use the
same bus by default. I suspect if we made PCI the default, they might
just always set the PCI-e flag just because.
There are hundreds of thousands if not millions of guests with existing
virtio-pci drivers. Forcing them to upgrade better have an extremely
good justification.
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.
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.
It also means we don't need to play games about sometimes enabling IO
bars and sometimes not.
Regards,
Anthony Liguori
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