Re: [PATCH 4/4] virtio-iommu: Clarify hot-unplug behavior

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Sat, Aug 12, 2023 at 03:25:10PM +0900, Akihiko Odaki wrote:
> > > By the way, crosvm's logic to detach endpoint on removal looks incorrect for
> > > me. A domain may have several endpoints attached, but the code looks like
> > > it's always destroying a domain whether there are other endpoints attached
> > > to the domain. I'm adding Zide Chen, who wrote the code according to git
> > > blame, and crosvm-dev@chromium.org to CC.
> > 
> > Link to this thread for more context:
> > https://lore.kernel.org/virtio-dev/20230803153238.541803-5-jean-philippe@linaro.org/
> > 
> > I thought crosvm rejected attaching multiple endpoints to one domain but I
> > think I misread. Rejecting multiple attach would be a straightforward fix
> > (it's allowed by the spec), though it would prevent assigning endpoints
> > that cannot be isolated from each others by the hardware (the driver won't
> > attach those to different domains, if it's made aware that they should be
> > in the same IOMMU group, for example if they are on a conventional PCI
> > bus).
> 
> Now we figured out an endpoint should not be detached from a domain without
> a request from the driver anyway so the code to detach an endpoint can be
> simply removed.

Yes, but I think the other detach path, when handling ATTACH or DETACH
requests, doesn't support domains with multiple endpoints attached either:

    // Currently, we only support detaching an endpoint if it is the only endpoint attached
    // to its domain.

But the ATTACH handler seems to accept attaching multiple endpoints to the
same domain?

Thanks,
Jean