[Xen-devel] Re: [PATCH 0/4] (Refactored) VHD Support Addition

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

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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