Re: [RFC PATCH 00/12] s390: virtio: support protected virtualization

[Cornelia Huck <cohuck@redhat.com>]

On Fri,  5 Apr 2019 01:16:10 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> Enhanced virtualization protection technology may require the use of
> bounce buffers for I/O. While support for this was built into the virtio
> core,  virtio-ccw wasn't changed accordingly.
> 
> Thus what needs to be done to bring virtio-ccw up to speed with respect
> to this is:
> * use some 'new' common virtio stuff
> * make sure that virtio-ccw specific stuff uses shared memory when
>   talking to the hypervisor (except communication blocks like ORB, these
>   are handled by the hypervisor)
> * make sure the DMA API does what is necessary to talk through shared
>   memory if we are a protected virtualization guest.
> * make sure the common IO layer plays along as well (airqs, sense).

It would be good to have a summary somewhere in the code (or
Documentation/) as to what needs the dma treatment and what doesn't,
for later reference. We don't want people to accidentally break things
(especially if they cannot refer to architecture documentation - or
will at least some of that be published?)

> 
> The series is structured in incremental fashion: some of the changes are
> overridden by following patches. The main reason why is that this is how I
> developed. But I think it ain't bad for the didactic and we are a bit more
> flexible with regards to throwing out some of the stuff in the end.

FWIW, I think reshuffling the patches in the next iteration would ease
review.

> 
> Important notes:
> 
> * This is an early (WIP) RFC that does not add any function to the
>   kernel at his stage, as the ultravisor interactions are left out.
>   The purpose is getting some early feedback ASAP.

I would like some comments from people who have experience with the dma
api.

> 
> * In future these patches will depend on some code interacting with the
>   ultravisor (WIP by Vasily and Janosch).
> 
> * The s390 names are by no means final, and are not properly explained. Should
>   not hamper understanding too much. If it does please ask.
> 
> * The existing naming in the common infrastructure (kernel internal
>   interfaces) is pretty much based on the AMD SEV terminology. Thus the
>   names aren't always perfect. There might be merit to changing these
>   names to more abstract ones. I did not put much thought into that at
>   the current stage.

If we can find some generic names that work well for everyone,
converting seems like a good idea. But following SEV is not that bad,
either (you'll probably find more people who have heard about SEV than
folks familiar with s390 ;)

> 
> 
> Testing:
> 
> Please use iommu_platform=on for any virtio devices you are going
> to test this code with (so virtio actually uses the DMA API).
> 
> Looking forward to your review or any other type of input.

I have now read through the whole series and commented in some places.
But I'd really like to see comments from others as well.