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

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Date:   Thu, 8 Sep 2022 15:28:22 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>, will@...nel.org,
        robin.murphy@....com, alex.williamson@...hat.com,
        suravee.suthikulpanit@....com, marcan@...can.st,
        sven@...npeter.dev, alyssa@...enzweig.io, robdclark@...il.com,
        dwmw2@...radead.org, baolu.lu@...ux.intel.com,
        mjrosato@...ux.ibm.com, gerald.schaefer@...ux.ibm.com,
        orsonzhai@...il.com, baolin.wang@...ux.alibaba.com,
        zhang.lyra@...il.com, thierry.reding@...il.com, vdumpa@...dia.com,
        jonathanh@...dia.com, jean-philippe@...aro.org, cohuck@...hat.com,
        tglx@...utronix.de, shameerali.kolothum.thodi@...wei.com,
        thunder.leizhen@...wei.com, christophe.jaillet@...adoo.fr,
        yangyingliang@...wei.com, jon@...id-run.com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, asahi@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-tegra@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        kevin.tian@...el.com
Subject: Re: [PATCH v6 1/5] iommu: Return -EMEDIUMTYPE for incompatible
 domain and device/group

On Wed, Sep 07, 2022 at 02:10:33PM -0300, Jason Gunthorpe wrote:
> Sure, rust has all sorts of nice things. But the kernel doesn't follow
> rust idioms, and I don't think this is a great place to start
> experimenting with them.

It is actually a great place to start experimenting. The IOMMU
interfaces are rather domain specific and if we get something wrong the
damage is limited to a few callers. There are APIs much more exposed in
the kernel which would be worse for that.

But anyway, I am not insisting on it.

> It has been 3 months since EMEDIUMTYPE was first proposed and 6
> iterations of the series, don't you think it is a bit late in the game
> to try to experiment with rust error handling idioms?

If I am not mistaken, I am the person who gets blamed when crappy IOMMU
code is sent upstream. So it is also up to me to decide in which state
and how close to merging a given patch series is an whether it is
already 'late in the game'.

> So, again, would you be happy with a simple 
> 
>  #define IOMMU_EINCOMPATIBLE_DEVICE xx
> 
> to make it less "re-using random error codes"?

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
	...

That would be much more intuitive than using something obscure like
EMEDIUMTYPE.

Regards,

	Joerg