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

[Jintack Lim]

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.

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?

I'm currently using Linux kernel 4.15, and Xen 4.10.1.

Thanks,
Jintack

[1] https://wiki.xenproject.org/wiki/Understanding_the_Virtualization_Spectrum

[Juergen Gross]

Post by Jintack Lim
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.

Post by Jintack Lim
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?


Juergen

[Jintack Lim]

Thanks Juergen,

Post by Juergen Gross

Post by Jintack Lim
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.

Post by Juergen Gross

Post by Jintack Lim
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

Post by Juergen Gross
Juergen