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

From: Nicolin Chen <nicolinc@nvidia.com>
To: Jason Gunthorpe <jgg@nvidia.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: <joro@8bytes.org>, <suravee.suthikulpanit@amd.com>,
	<will@kernel.org>, <robin.murphy@arm.com>, <robdclark@gmail.com>,
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	<agross@kernel.org>, <bjorn.andersson@linaro.org>,
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Subject: Re: [PATCH 4/5] iommu: Regulate errno in ->attach_dev callback functions
Date: Wed, 14 Sep 2022 10:58:27 -0700	[thread overview]
Message-ID: <YyIWQ6rX6AR9KX5E@Asurada-Nvidia> (raw)
In-Reply-To: <YyGjtsB2Yq4fQICS@myrica>

On Wed, Sep 14, 2022 at 10:49:42AM +0100, Jean-Philippe Brucker wrote:
> External email: Use caution opening links or attachments
> 
> 
> 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.

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

> > > 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?