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

Michael S. Tsirkin
Wed, 05 Jun 2013 19:52:16 -0000

On Wed, Jun 05, 2013 at 10:43:17PM +0300, Michael S. Tsirkin
wrote:
> On Wed, Jun 05, 2013 at 01:57:16PM -0500, Anthony Liguori wrote:
> > "Michael S. Tsirkin" <mst at 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).
>
>
> Let me add 3 other issues which I mentioned and you seem to miss:
>
> 3) architectures which don't have fast access to IO ports, exist
>    virtio does not work there ATM
>
> 4) setups with many PCI bridges exist and have the same issue
>    as PCI express. virtio does not work there ATM
>
> 5) On x86, even with nested page tables, firmware only decodes
>    the page address on an invalid PTE, not the data. You need to
>    emulate the guest to get at the data. Without
>    nested page tables, we have to do page table walk and emulate
>    to get both address and data. Since this is how MMIO
>    is implemented in kvm on x86, MMIO is much slower than PIO
>    (with nested page tables by a factor of >2, did not test without).

Oh I forgot:

6) access to MMIO BARs is painful in the BIOS environment
   so BIOS would typically need to enable IO for the boot device.



> > 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),
>
> We can't do this - it will hurt performance.
>
> >    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.
>
> Why do you think 2 == 3? 2 changes default behaviour. 3 does not.
>
> > 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.
>
> This sounds very strange. But let's assume you are right for
> the sake of the argument ...
>
> > 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.
>
> I don't think how this changes the situation. libvirt still need
> to set policy and decide which device to use.
>
> > 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.
>
> Only because the chance it's 100% compatible on the software level is
0.
> It always has some hardware specific quirks.
> No such excuse here.
>
> > It also means we don't need to play games about sometimes enabling
IO
> > bars and sometimes not.
>
> This last paragraph is wrong, it ignores the issues 3) to 5)
> I added above.
>
> If you do take them into account:
> 	- there are reasons to add MMIO BAR to PCI,
> 	  even without PCI express
> 	- we won't be able to drop IO BAR from virtio
>
> > Regards,
> >
> > Anthony Liguori
> >
> > >
> > >
> > > --
> > > MST
> > > --
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