Gerd Hoffmann wrote:
Avi Kivity wrote:
You are
probably better off designing something that is PV specific instead of
shoehorning it in to fit a different model (at least for the things I
have in mind).
Well, if we design our pv devices to look like hardware, they will fit
quite well. Both to the guest OS and to user's expectations.
Disclaimer: Havn't looked at the virtio code much.
I think we should keep the door open for both models and don't nail the
virtio infrastructure to one of them.
For pure pv devices I don't see the point in trying to squeeze it into
the PCI model. Also s390 has no PCI, so there effecticely is no way
around that, we must be able have some pure virtual bus like xenbus.
I don't really agree with this assessment. There is no performance
advantage to using a pure virtual bus. If you have a pure pv device
that looks and act like a PCI device, besides the obvious advantage of
easy portability to other guest OSes (since everything support PCI, but
porting XenBus--event to Linux 2.4.x was a royal pain), it is also very
easy to support the device on other VMMs.
For instance, the PCI device that I just posted would allow virtio
devices to be used trivially with HVM on Xen. In fact, once the
backends are complete and merged into QEMU, the next time Xen rebases
QEMU they'll get the virtio PV-on-HVM drivers for free. To me, that's a
pretty significant advantage.
Uhm, well, yea. Guess you are refering to the pv-on-hvm drivers. Been
there, dealt with it. What exactly do you think is messy there?
IMHO the most messy thing is the boot problem. hvm bios can't deal with
pv disks, so you can't boot with pv disks only. "fixed" by having the
(boot) disk twice in the system, once via emulated ide, once as pv disk.
Ouch.
I have actually addressed this problem with a PV option rom for QEMU. I
expect to get time to submit the QEMU patches by the end of the year.
See http://hg.codemonkey.ws/extboot
Regards,
Anthony Liguori
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