Re: [PATCH v5 0/5] cover-letter: Simplify vfio_iommu_type1 attach/detach routine

Alex Williamson
2022-07-06T18:03:40Z

On Wed, 6 Jul 2022 10:53:52 -0700
Nicolin Chen <nicolinc@nvidia.com> wrote:

> On Wed, Jul 06, 2022 at 11:42:17AM -0600, Alex Williamson wrote:
>
> > On Fri, 1 Jul 2022 14:44:50 -0700
> > Nicolin Chen <nicolinc@nvidia.com> wrote:
> >
> > > This is a preparatory series for IOMMUFD v2 patches. It enforces error
> > > code -EMEDIUMTYPE in iommu_attach_device() and iommu_attach_group() when
> > > an IOMMU domain and a device/group are incompatible. It also drops the
> > > useless domain->ops check since it won't fail in current environment.
> > >
> > > These allow VFIO iommu code to simplify its group attachment routine, by
> > > avoiding the extra IOMMU domain allocations and attach/detach sequences
> > > of the old code.
> > >
> > > Worths mentioning the exact match for enforce_cache_coherency is removed
> > > with this series, since there's very less value in doing that as KVM will
> > > not be able to take advantage of it -- this just wastes domain memory.
> > > Instead, we rely on Intel IOMMU driver taking care of that internally.
> > >
> > > This is on github:
> > > https://github.com/nicolinc/iommufd/commits/vfio_iommu_attach
> >
> > How do you foresee this going in, I'm imagining Joerg would merge the
> > first patch via the IOMMU tree and provide a topic branch that I'd
> > merge into the vfio tree along with the remaining patches. Sound
> > right? Thanks,
>
> We don't have any build dependency between the IOMMU change and
> VFIO changes, yet, without the IOMMU one, any iommu_attach_group()
> failure now would be a hard failure without a chance falling back
> to a new_domain, which is slightly different from the current flow.
>
> For a potential existing use case that relies on reusing existing
> domain, I think it'd be safer to have Joerg acking the first change
> so you merge them all? Thank!

Works for me, I'll look for buy-in + ack from Joerg. Thanks,

Alex