Re: [Xen-devel] Running Driver Domain in different mode and hypervisor

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Feb 27, 2019 at 12:26:19AM +0000, tosher 1 wrote:
> Hi Guys,
> Lately, I have been trying to play with Xen Driver Domain. I have been able 
> to make it work where both the  Driver Domain OS and the guest OS are run on 
> paravirtualized (PV) machine. However, it doesn't work when any of them are 
> hardware virtualized machine (HVM). Therefore, I have the following questions 
> now.
> 1. Is it possible to have the Driver Domain and guests, who are using the 
> corresponding drivers, run on virtual machines using other than the PV mode, 
> like HVM or PVHVM? If yes, then what are the possible combinations?

Driver domains should work for all domain types (PV, PVH, HVM,
PVHVM...).

> 2. Is it possible to use virtIO for paravirtualization where the underlying 
> hypervisor is Xen?

AFAIK, yes, with caveats [0].

> If yes, then can we run Driver Domain using virtIO?

Likely not, at least not without breaking Xen security model. You
would have to grant the driver domain full access to the memory of the
other end(s) (the domain(s) running the frontends). That's because
VirtIO doesn't have support grants [1], and currently assumes it has
full access to the memory of the domain running the frontend.

> 3. Is it possible to have a Driver Domain where the underlying hypervisor is 
> KVM instead of Xen?

I don't think so, KVM doesn't have anything akin to grants, so you
would have to allow the driver domain full access to other domains
memory, which basically removes a lot of benefits from the concept of
driver domains.

> The last question may not be directly related to Xen. However, I am tempted 
> to ask as good things like PV was first introduced in Xen then others 
> hypervisors adapted this design.  

I'm not sure I understand this last question, sorry.

Do you mean to ask why other hypervisors haven't implemented PV
support?

Roger.

[0] https://wiki.xenproject.org/wiki/QEMU_Upstream#VirtIO
[1] https://wiki.xen.org/wiki/Grant_Table

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