Re: [Xen-devel] xen/xen vs xen/kvm nesting with pv drivers

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/09/16 13:47, Anthony Wright wrote:
> I tried to install Xen (4.7.0 with linux 4.7.2 Dom0) on an AWS virtual 
> machine and it failed because while AWS uses Xen it requires that you use the 
> PVHVM network driver. I then tried to install Xen on a Google Cloud virtual 
> machine and despite also requiring you to use PV drivers, that succeeded 
> because Google Cloud uses KVM.
>
> I think this means that if you nest Xen in KVM you can use high performance 
> drivers, but if you nest Xen in Xen you have to use slower drivers, which 
> seems to be the wrong way around!
>
> I'd like to be able to install Xen on an AWS virtual machine, and wondered 
> what are the challenges to getting the pv drivers working in a nested 
> environment. Is this a problem with the Dom0 kernel only expecting there to 
> be a single XenStore, or is there also a problem in Xen?

Nesting Xen inside Xen and getting high-speed drivers at L1 is a hard
problem, which is why noone has tackled it yet.

The problems all revolve around L1's dom0.  It can't issue hypercalls to
L0, meaning that it cant find or connect the xenstore ring.  Even if it
could, there is the problem of multiple xenstores, which doesn't fit in
the current architecture.

It would be lovely if someone would work on this, but it is a very large
swamp.

~Andrew

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