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

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Date:   Thu, 30 Jun 2022 09:57:28 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Robin Murphy <robin.murphy@....com>,
        Nicolin Chen <nicolinc@...dia.com>,
        Yong Wu <yong.wu@...iatek.com>
CC:     Jason Gunthorpe <jgg@...dia.com>,
        Baolu Lu <baolu.lu@...ux.intel.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "jordan@...micpenguin.net" <jordan@...micpenguin.net>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "will@...nel.org" <will@...nel.org>,
        "alyssa@...enzweig.io" <alyssa@...enzweig.io>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "zhang.lyra@...il.com" <zhang.lyra@...il.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "jonathanh@...dia.com" <jonathanh@...dia.com>,
        "yangyingliang@...wei.com" <yangyingliang@...wei.com>,
        "orsonzhai@...il.com" <orsonzhai@...il.com>,
        "gerald.schaefer@...ux.ibm.com" <gerald.schaefer@...ux.ibm.com>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "christophe.jaillet@...adoo.fr" <christophe.jaillet@...adoo.fr>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "marcan@...can.st" <marcan@...can.st>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baolin.wang7@...il.com" <baolin.wang7@...il.com>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>
Subject: RE: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain
 and device/group

> From: Robin Murphy <robin.murphy@....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. 😊