[PATCH 4/5] iommu: Regulate errno in ->attach_dev callback functions

[Jean-Philippe Brucker]

On Wed, Sep 14, 2022 at 06:11:06AM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 13, 2022 at 01:27:03PM +0100, Jean-Philippe Brucker wrote:
> > I think in the future it will be too easy to forget about the constrained
> > return value of attach() while modifying some other part of the driver,
> > and let an external helper return EINVAL. So I'd rather not propagate ret
> > from outside of viommu_domain_attach() and finalise().
> 
> Fortunately, if -EINVAL is wrongly returned it only creates an
> inefficiency, not a functional problem. So we do not need to be
> precise here.

Ah fair. In that case the attach_dev() documentation should indicate that
EINVAL is a hint, so that callers don't rely on it (currently words "must"
and "exclusively" indicate that returning EINVAL for anything other than
device-domain incompatibility is unacceptable). The virtio-iommu
implementation may well return EINVAL from the virtio stack or from the
host response.

Thanks,
Jean

> 
> > Since we can't guarantee that APIs like virtio or ida won't ever return
> > EINVAL, we should set all return values:
> 
> I dislike this alot, it squashes all return codes to try to optimize
> an obscure failure path :(
> 
> Jason