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

On 06/09/2016 14:05, Andrew Cooper wrote:
> 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
Does the L1's Dom0 have to issue the hypercalls directly? Would it be
possible to get the L1's Dom0 to issue the request to the L1 hypervisor
and that to call the L0 hypervisor? This would seem to fit the current
architecture fairly closely. (Sorry if I've got the terminology wrong)

Regarding multiple XenStores, I appreciate there would be significant
problems, but you'd only have a maximum of two XenStores, one for the
xenback drivers (the current XenStore) and one for the xenfront drivers
(that talks to the parent hypervisor).


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