Anthony Liguori wrote:
Zachary Amsden wrote:
Jeremy Fitzhardinge wrote:
Anthony Liguori wrote:
I don't agree that having paravirt_ops within a normal module is all
that useful. By the time modules can be loaded, the kernel has
completely booted. There should only be a handful of paravirt_ops
implementations and they aren't large so I don't think there's a big
size savings either.
It doesn't seem terribly valuable to me either. But Zach is talking
about something very similar to the kvm case, where you have a fully
virtualized environment (with hardware support), but then you load a
module containing paravirtualized helpers at some late stage which
makes
things more efficient but isn't required for functional correctness.
Yes, the value isn't the space savings - it's the ability to include
paravirtualized driver support for Xen, KVM, VMI, lhype - which need
not be compiled in, but can now be modules in a ramdisk. The goal is
minimal effort for a single bootable image which works across native
and all virtualization environments.
But what's the value in having it not in the kernel? Let's take Xen
and lhype out of the picture because it clearly has to be there for
them. You have a little less in the kernel now but then your kernel
boots more slowly. There's already a noticable difference in
boot-time with the KVM paravirt_ops implementation. I imagine there
is for VMI too.
If it isn't compiled in the core kernel, then a distro need not do
anything special to distribute VMI or KVM support - other than compile
support for paravirt-ops. Then the paravirt-ops module can be installed
along with the guest tools and drivers, but need not be on install media.
Basically, it just makes it easier on distributors and allows any old
kernel with paravirt-ops module support to run on any modern, new
hypervisor - that might not have even existed at the time the distro was
created.
In the case of KVM, the paravirt_ops implementation is orthogonal to
paravirt device drivers. A PV device driver can happily exist even if
the paravirt_ops backend isn't activated. This is assuming that
hypercalls aren't used btw. If hypercalls are desirable to use, then
the paravirt_ops backend would have to EXPORT_GPL the hypercall
interface. I imagine returning a specific errno would suffice.
I'm mostly in agreement on that - although making dual hypercall / I/O
emulated drivers is a bit more work.
Zach
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