Re: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group

From: Yong Wu <yong.wu@mediatek.com>
To: Nicolin Chen <nicolinc@nvidia.com>, Baolu Lu <baolu.lu@linux.intel.com>
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Subject: Re: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
Date: Fri, 24 Jun 2022 13:38:58 +0800	[thread overview]
Message-ID: <8a5e9c81ab1487154828af3ca21e62e39bcce18c.camel@mediatek.com> (raw)
In-Reply-To: <YrUk8IINqDEZLfIa@Asurada-Nvidia>

On Thu, 2022-06-23 at 19:44 -0700, Nicolin Chen wrote:
> On Fri, Jun 24, 2022 at 09:35:49AM +0800, Baolu Lu wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > On 2022/6/24 04:00, Nicolin Chen wrote:
> > > diff --git a/drivers/iommu/mtk_iommu_v1.c
> > > b/drivers/iommu/mtk_iommu_v1.c
> > > index e1cb51b9866c..5386d889429d 100644
> > > --- a/drivers/iommu/mtk_iommu_v1.c
> > > +++ b/drivers/iommu/mtk_iommu_v1.c
> > > @@ -304,7 +304,7 @@ static int mtk_iommu_v1_attach_device(struct
> > > iommu_domain *domain, struct device
> > >       /* Only allow the domain created internally. */
> > >       mtk_mapping = data->mapping;
> > >       if (mtk_mapping->domain != domain)
> > > -             return 0;
> > > +             return -EMEDIUMTYPE;
> > > 
> > >       if (!data->m4u_dom) {
> > >               data->m4u_dom = dom;
> > 
> > This change looks odd. It turns the return value from success to
> > failure. Is it a bug? If so, it should go through a separated fix
> > patch.

Thanks for the review:)

> 
> Makes sense.
> 
> I read the commit log of the original change:
> 
https://lore.kernel.org/r/1589530123-30240-1-git-send-email-yong.wu@mediatek.com
> 
> It doesn't seem to allow devices to get attached to different
> domains other than the shared mapping->domain, created in the
> in the mtk_iommu_probe_device(). So it looks like returning 0
> is intentional. Though I am still very confused by this return
> value here, I doubt it has ever been used in a VFIO context.

It's not used in VFIO context. "return 0" just satisfy the iommu
framework to go ahead. and yes, here we only allow the shared "mapping-
>domain" (All the devices share a domain created internally).

thus I think we should still keep "return 0" here.

Thanks:)

> 
> Young, would you please give us some input?
> 
> Overall, I feel it's better to play it safe here by dropping
> this part. If we later confirm there is a need to fix it, we
> will do that in a separate patch anyway.
> 
> Thanks
> Nic