Re: [RFC PATCH 00/21] Add SMMUv3 Stage 1 Support for XEN guests

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sat, 3 Dec 2022, Julien Grall wrote:
> On 01/12/2022 16:02, Rahul Singh wrote:
> > This patch series is sent as RFC to get the initial feedback from the
> > community. This patch series consists of 21 patches which is a big number
> > for
> > the reviewer to review the patches but to understand the feature end-to-end
> > we
> > thought of sending this as a big series. Once we will get initial feedback,
> > we
> > will divide the series into a small number of patches for review.
> 
> From the cover letter, it is not clear to me what sort of input you are
> expecting for the RFC. Is this about the design itself?
> 
> If so, I think it would be more helpful to write an high level document on how
> you plan to emulate the vIOMMU in Xen. So there is one place to
> read/agree/verify rather than trying to collate all the information from the
> 20+ patches.
> 
> Briefly skimming through I think the main things that need to be addressed in
> order of priority:
>   - How to secure the vIOMMU
>   - 1 vs multiple vIOMMU
> 
> The questions are very similar to the vITS because the SMMUv3 is based on a
> queue. And given you are selling this feature as a security one, I don't think
> we can go forward with the review without any understanding/agreement on what
> needs to be implemented in order to have a safe/secure vIOMMU.

I think we are all aligned here, but let me try to clarify further.

As the vIOMMU is exposed to the guest, and exposing a queue-based
interface to the guest is not simple, it would be good to clarify in a
document the following points:

- how is the queue exposed to the guest
- how are guest-inputs sanitized
- how do the virtual queue resources map to the physical queue
  resources
- lifecycle of the resource mappings
- any memory allocations triggered by guest actions and their lifecycle

It is difficult to extrapole these details from 21 patches. Having these
key detailed written down in the 0/21 email would greatly help with the
review. It would make the review go a lot faster.