Re: CONFIG_XEN_VIRTIO{_FORCE_GRANT} interferes with nested virt

[Juergen Gross <jgross@xxxxxxxx>]

On 05.10.22 15:25, Marek Marczykowski-Górecki wrote:
> On Wed, Oct 05, 2022 at 02:57:01PM +0200, Juergen Gross wrote:
> > On 05.10.22 14:41, Marek Marczykowski-Górecki wrote:
> > > Hi,
> > >
> > > When booting Xen with Linux dom0 nested under KVM,
> > > CONFIG_XEN_VIRTIO_FORCE_GRANT=y makes it unable to use virtio devices
> > > provided by L0 hypervisor (KVM with qemu). With PV dom0, grants are
> > > required for virtio even if just CONFIG_XEN_VIRTIO is enabled.
> > >
> > > This is probably uncommon corner case, but one that has bitten me in my
> > > CI setup... I think Xen should set smarter
> > > virtio_require_restricted_mem_acc(), that enforces it only for devices
> > > really provided by another Xen VM (not by the "outer host"), but I'm not
> > > sure how that could be done. Any ideas?
> > It should be possible to add a boot parameter for that purpose. Using it
> > would open a security hole, though (basically like all PCI passthrough to
> > PV guests).
> What about excluding just dom0? At least currently, there is no way for
> dom0 to see virtio devices provided by another Xen domU.
Even not via hotplug?

> Something like this:
> ---8<---
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 9b1a58dda935..6ac32b0b3720 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -111,7 +111,7 @@ static DEFINE_PER_CPU(struct tls_descs, shadow_tls_desc);
>   static void __init xen_pv_init_platform(void)
>   {
>          /* PV guests can't operate virtio devices without grants. */
> -       if (IS_ENABLED(CONFIG_XEN_VIRTIO))
> +       if (IS_ENABLED(CONFIG_XEN_VIRTIO) && !xen_initial_domain())
>                  virtio_set_mem_acc_cb(virtio_require_restricted_mem_acc);
>   
>          populate_extra_pte(fix_to_virt(FIX_PARAVIRT_BOOTMAP));
> ---8<---
>
> This BTW raises also a question what will happen on Xen nested inside
> Xen, when L0 will provide virtio devices to L1. Grants set by L1 dom0
> wouldn't work on L0, no? Or maybe this is solved already for pv-shim
> case?
This is a similar problem as with normal Xen PV devices.

You will need either a simple grant passthrough like with pv-shim (enabling
such devices for one guest in L1 only), or you need a grant multiplexer in L1 
Xen in case you want to be able to have multiple guests in L1 being able to
use L0 PV devices.


Juergen

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