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xen-devel
[Xen-devel] Re: [PATCH 0/4] (Refactored) VHD Support Addition
Steve Ofsthun wrote:
QEMU already supports VHD (it calls it VPC) and supported it long before
Microsoft released any specs. QEMU supported VMDK before VMware release
the specs for that too.
This code was developed in the Xen 3.0.2 days when the integrated qemu had
no vpc support. Our original code was targeted at PV on HVM drivers using
blocktap. We needed to support the same format unaccelerated and so the
qemu integration was also done.
I suspect there are really two possible ways to go here:
1) Add generic support to mount blktap disks to QEMU via block-add in
dom0 and then apply your stuff to blktap.
2) Just enhance the VPC in QEMU to cover anything it doesn't now. That
doesn't really address blktap but that's the right thing to do from the
QEMU perspective.
Adding a second block-vhd to QEMU is really a bad idea from a
maintainability perspective. #1 is much better since it avoids that
problem.
Regards,
Anthony Liguori
Is there a particular feature of VHD/VPC that your code adds that QEMU
didn't already support?
Not having any experience with VPC, I couldn't tell you. I can say that we
tried to adhere to the Microsoft spec rather than reverse engineering the
format from examples. Perusing the VPC code, there are many references to
unknown fields within the format.
Is there a complete and concise summary of the VPC feature coverage?
Thanks,
Steve
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