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

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Robin Murphy <robin.murphy@arm.com>,
	Nicolin Chen <nicolinc@nvidia.com>,
	Yong Wu <yong.wu@mediatek.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>,
	Baolu Lu <baolu.lu@linux.intel.com>,
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	"cohuck@redhat.com" <cohuck@redhat.com>,
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	"will@kernel.org" <will@kernel.org>,
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	"jean-philippe@linaro.org" <jean-philippe@linaro.org>,
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	"iommu@lists.linux-foundation.org"
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Subject: RE: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
Date: Thu, 30 Jun 2022 09:57:28 +0000	[thread overview]
Message-ID: <BN9PR11MB527663460076DA4BC8D34D3F8CBA9@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: <e5799215-8b55-90a8-7ca4-35f85ffb5969@arm.com>

> From: Robin Murphy <robin.murphy@arm.com>
> Sent: Thursday, June 30, 2022 4:22 PM
> 
> On 2022-06-29 20:47, Nicolin Chen wrote:
> > On Fri, Jun 24, 2022 at 03:19:43PM -0300, Jason Gunthorpe wrote:
> >> On Fri, Jun 24, 2022 at 06:35:49PM +0800, Yong Wu wrote:
> >>
> >>>>> 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).
> >>
> >> What part of the iommu framework is trying to attach a domain and
> >> wants to see success when the domain was not actually attached ?
> >>
> >>>> What prevent this driver from being used in VFIO context?
> >>>
> >>> Nothing prevent this. Just I didn't test.
> >>
> >> This is why it is wrong to return success here.
> >
> > Hi Yong, would you or someone you know be able to confirm whether
> > this "return 0" is still a must or not?
> 
>  From memory, it is unfortunately required, due to this driver being in
> the rare position of having to support multiple devices in a single
> address space on 32-bit ARM. Since the old ARM DMA code doesn't
> understand groups, the driver sets up its own canonical
> dma_iommu_mapping to act like a default domain, but then has to politely
> say "yeah OK" to arm_setup_iommu_dma_ops() for each device so that they
> do all end up with the right DMA ops rather than dying in screaming
> failure (the ARM code's per-device mappings then get leaked, but we
> can't really do any better).
> 
> The whole mess disappears in the proper default domain conversion, but
> in the meantime, it's still safe to assume that nobody's doing VFIO with
> embedded display/video codec/etc. blocks that don't even have reset drivers.
> 

Probably above is worth a comment in mtk code so we don't need
always dig it out from memory when similar question arises in the
the future. 😊