Anthony Liguori wrote:
Another point is that virtio still has a lot of leading zeros in its
mileage counter. We need to keep things flexible and learn from
others as much as possible, especially when talking about the ABI.
Yes, after thinking about it over holiday, I agree that we should at
least introduce a virtio-pci feature bitmask. I'm not inclined to
attempt to define a hypercall ABI or anything like that right now but
having the feature bitmask will at least make it possible to do such a
thing in the future.
No, definitely not define a hypercall ABI. The feature bit should say
"this device understands a hypervisor-specific way of kicking. consult
your hypervisor manual and cpuid bits for further details. should you
not be satisfied with this method, port io is still available".
I'm wary of introducing the notion of hypercalls to this device
because it makes the device VMM specific. Maybe we could have the
device provide an option ROM that was treated as the device "BIOS"
that we could use for kicking and interrupt acking? Any idea of how
that would map to Windows? Are there real PCI devices that use the
option ROM space to provide what's essentially firmware?
Unfortunately, I don't think an option ROM BIOS would map well to
other architectures.
The BIOS wouldn't work even on x86 because it isn't mapped to the
guest address space (at least not consistently), and doesn't know the
guest's programming model (16, 32, or 64-bits? segmented or flat?)
Xen uses a hypercall page to abstract these details out. However, I'm
not proposing that. Simply indicate that we support hypercalls, and
use some layer below to actually send them. It is the responsibility
of this layer to detect if hypercalls are present and how to call them.
Hey, I think the best place for it is in paravirt_ops. We can even
patch the hypercall instruction inline, and the driver doesn't need
to know about it.
Yes, paravirt_ops is attractive for abstracting the hypercall calling
mechanism but it's still necessary to figure out how hypercalls would
be identified. I think it would be necessary to define a virtio
specific hypercall space and use the virtio device ID to claim subspaces.
For instance, the hypercall number could be (virtio_devid << 16) |
(call number). How that translates into a hypercall would then be
part of the paravirt_ops abstraction. In KVM, we may have a single
virtio hypercall where we pass the virtio hypercall number as one of
the arguments or something like that.
If we don't call it a hypercall, but a virtio kick operation, we don't
need to worry about the hypercall number or ABI. It's just a function
that takes an argument that's implemented differently by every
hypervisor. The default implementation can be a pio operation.
Make it appear as a pci function? (though my feeling is that
multiple mounts should be different devices; we can then hotplug
mountpoints).
We may run out of PCI slots though :-/
Then we can start selling virtio extension chassis.
:-) Do you know if there is a hard limit on the number of devices on
a PCI bus? My concern was that it was limited by something stupid
like an 8-bit identifier.
IIRC pci slots are 8-bit, but you can have multiple buses, so
effectively 16 bits of device address space (discounting functions which
are likely not hot-pluggable).
--
error compiling committee.c: too many arguments to function
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