Re: Virtio in Xen on Arm (based on IOREQ concept)

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

On Mon, Jul 20, 2020 at 10:40:40AM +0100, Julien Grall wrote:
> 
> 
> On 20/07/2020 10:17, Roger Pau Monné wrote:
> > On Fri, Jul 17, 2020 at 09:34:14PM +0300, Oleksandr wrote:
> > > On 17.07.20 18:00, Roger Pau Monné wrote:
> > > > On Fri, Jul 17, 2020 at 05:11:02PM +0300, Oleksandr Tyshchenko wrote:
> > > > Do you have any plans to try to upstream a modification to the VirtIO
> > > > spec so that grants (ie: abstract references to memory addresses) can
> > > > be used on the VirtIO ring?
> > > 
> > > But VirtIO spec hasn't been modified as well as VirtIO infrastructure in 
> > > the
> > > guest. Nothing to upsteam)
> > 
> > OK, so there's no intention to add grants (or a similar interface) to
> > the spec?
> > 
> > I understand that you want to support unmodified VirtIO frontends, but
> > I also think that long term frontends could negotiate with backends on
> > the usage of grants in the shared ring, like any other VirtIO feature
> > negotiated between the frontend and the backend.
> > 
> > This of course needs to be on the spec first before we can start
> > implementing it, and hence my question whether a modification to the
> > spec in order to add grants has been considered.
> The problem is not really the specification but the adoption in the
> ecosystem. A protocol based on grant-tables would mostly only be used by Xen
> therefore:
>    - It may be difficult to convince a proprietary OS vendor to invest
> resource on implementing the protocol
>    - It would be more difficult to move in/out of Xen ecosystem.
> 
> Both, may slow the adoption of Xen in some areas.

Right, just to be clear my suggestion wasn't to force the usage of
grants, but whether adding something along this lines was in the
roadmap, see below.

> If one is interested in security, then it would be better to work with the
> other interested parties. I think it would be possible to use a virtual
> IOMMU for this purpose.

Yes, I've also heard rumors about using the (I assume VirtIO) IOMMU in
order to protect what backends can map. This seems like a fine idea,
and would allow us to gain the lost security without having to do the
whole work ourselves.

Do you know if there's anything published about this? I'm curious
about how and where in the system the VirtIO IOMMU is/should be
implemented.

Thanks, Roger.