Hi Robin,

On Wed, Sep 14, 2022 at 08:53:07PM +0100, Robin Murphy wrote:

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On 2022-09-14 18:58, Nicolin Chen wrote:

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On Wed, Sep 14, 2022 at 10:49:42AM +0100, Jean-Philippe Brucker wrote:

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On Wed, Sep 14, 2022 at 06:11:06AM -0300, Jason Gunthorpe wrote:

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On Tue, Sep 13, 2022 at 01:27:03PM +0100, Jean-Philippe Brucker wrote:

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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.

How about this?

+ * * EINVAL    - mainly, device and domain are incompatible, or something went
+ *               wrong with the domain. It's suggested to avoid kernel prints
+ *               along with this errno. And it's better to convert any EINVAL
+ *               returned from kAPIs to ENODEV if it is device-specific, or to
+ *               some other reasonable errno being listed below

FWIW, I'd say something like:

"The device and domain are incompatible. If this is due to some previous
configuration of the domain, drivers should not log an error, since it
is legitimate for callers to test reuse of an existing domain.
Otherwise, it may still represent some fundamental problem."

OK. I will use this narrative.

And then at the public interfaces state it from other angle:

"The device and domain are incompatible. If the domain has already been
used or configured in some way, attaching the same device to a different
domain may be expected to succeed. Otherwise, it may still represent
some fundamental problem."

I assume this should go to kdocs of iommu_attach_device/group().

[ and to save another mail, I'm not sure copying the default comment for
ENOSPC is all that helpful either - what is "space" for something that
isn't a storage device? I'd guess limited hardware resources in some
form, but in the IOMMU context, potential confusion with address space
is maybe a little too close for comfort? ]

How about "non-ENOMEM type of resource allocation failure"?

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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 :(

Hmm...should I revert all the driver changes back to this version?

Yeah, I don't think we need to go too mad here. Drivers shouldn't emit
their *own* -EINVAL unless appropriate, but if it comes back from some
external API then that implies something's gone unexpectedly wrong
anyway - maybe it's a transient condition and a subsequent different
attach might actually work out OK? We can't really say in general.

OK. Then there's even no need to convert EINVAL to ENODEV.

Besides, if the driver sees an error which implies it's done something
wrong itself, it probably shouldn't be trusted to try to reason about it
further. The caller can handle any error as long as we set their
expectations correctly.

Yea. As Jason remarked, a wrongly returned EINVAL would make things
a bit inefficient: VFIO/IOMMUFD would keep trying attach_dev() with
its existing domain list and a new domain but fail all of them.

I will change things in v2 back to this 2-patch version, and maybe
limit a bit further the changes in the first NODEV patch.

Thanks!
Nic

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