Re: [Xen-devel] How to use PV drivers with fully virtualized HVM on x86?

[Jintack Lim <jintack@xxxxxxxxxxxxxxx>]

Thanks Juergen,

On Wed, Oct 24, 2018 at 10:30 AM Juergen Gross <jgross@xxxxxxxx> wrote:
>
> On 24/10/2018 16:16, Jintack Lim wrote:
> > Hi,
> >
> > I'm interested running HVM DomU only with PV drivers (network and
> > disk). Enabling CONFIG_XEN_NETDEV_FRONTEND and stuff didn't make DomU
> > see the PV devices until I enabled CONFIG_XEN_PVHVM.
>
> Right, that's how it is done in modern Linux kernels.

Thanks for the confirmation.

>
> > But by looking at the virtualization spectrum[1], it seems possible to
> > do "HVM with PV drivers", but I couldn't find any guide (i.e.
> > kernel/xen configuration) to do that. Could anyone shed some light on
> > this?
>
> Disabling most of the PV interfaces other than those needed for using
> PV devices isn't supported in Linux since several years now.
>
> Any reason why you want to do that?

I was testing nested virtualization - Xen on KVM. I see network
performance degradation with PVHVM DomU in nested VM compared to a
DomU without PVHVM in nested VM. I'm using a physical NIC device
assigned to the DomU for both cases.

It's quite unusual setup, but I just wonder what causes performance
drop with PVHVM even though I'm not using PV network device in DomU. I
guess it might be some other stuff, but not sure what they are. So I
wanted to configure DomU to exactly have PV devices and see what
happens.

Thanks,
Jintack

>
>
> Juergen
>


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