RE: [PATCH v6 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group

Tian, Kevin
2022-09-09T05:00:46Z

> From: Nicolin Chen <nicolinc@nvidia.com>
> Sent: Friday, September 9, 2022 11:17 AM
>
> On Thu, Sep 08, 2022 at 01:14:42PM -0300, Jason Gunthorpe wrote:
>
> > > I am wondering if this can be solved by better defining what the return
> > > codes mean and adjust the call-back functions to match the definition.
> > > Something like:
> > >
> > > -ENODEV : Device not mapped my an IOMMU
> > > -EBUSY : Device attached and domain can not be changed
> > > -EINVAL : Device and domain are incompatible
> > > ...
> >
> > Yes, this was gone over in a side thread the pros/cons, so lets do
> > it. Nicolin will come with something along these lines.
>
> I have started this effort by combining this list and the one from
> the side thread:
>
> @@ -266,6 +266,13 @@ struct iommu_ops {
> /**
> * struct iommu_domain_ops - domain specific operations
> * @attach_dev: attach an iommu domain to a device
> + * Rules of its return errno:
> + * ENOMEM - Out of memory
> + * EINVAL - Device and domain are incompatible
> + * EBUSY - Device is attached to a domain and cannot be changed

With this definition then probably @attach_dev should not return -EBUSY
at all given it's already checked in the start of __iommu_attach_group():

if (group->domain && group->domain != group->default_domain &&
group->domain != group->blocking_domain)
return -EBUSY;

> + * ENODEV - Device or domain is messed up: device is not mapped
> + * to an IOMMU, no domain can attach, and etc.

if domain is messed up then should return -EINVAL given using another domain
might just work. IMHO here -ENODEV should only cover device specific problems
preventing this device from being attached to by any domain.

> + * <others> - Same behavior as ENODEV, use is discouraged

didn't get the "Same behavior" part. Does it suggest all other errnos should
be converted to ENODEV?

btw what about -ENOSPC? It's sane to allocate some resource in the attach
path while the resource might be not available, e.g.:

intel_iommu_attach_device()
domain_add_dev_info()
domain_attach_iommu():

int num, ret = -ENOSPC;
...
ndomains = cap_ndoms(iommu->cap);
num = find_first_zero_bit(iommu->domain_ids, ndomains);
if (num >= ndomains) {
pr_err("%s: No free domain ids\n", iommu->name);
goto err_unlock;
}

As discussed in a side thread a note might be added to exempt calling
kAPI outside of the iommu driver.

> * @detach_dev: detach an iommu domain from a device
> * @map: map a physically contiguous memory region to an iommu domain
> * @map_pages: map a physically contiguous set of pages of the same size
> to
>
> I am now going through every single return value of ->attach_dev to
> make sure the list above applies. And I will also incorporate things
> like Robin's comments at the AMD IOMMU driver.
>
> And if the change occurs to be bigger, I guess that separating it to
> be an IOMMU series from this VFIO one might be better.
>
> Thanks
> Nic