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Date: Mon, 24 Jul 2017 11:16:21 -0600
From: Alex Williamson <alex.williamson@redhat.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Will Deacon <will.deacon@arm.com>,
        Anup Patel <anup.patel@broadcom.com>, Joerg Roedel <joro@8bytes.org>,
        Baptiste Reynal <b.reynal@virtualopensystems.com>,
        Scott Branden <sbranden@broadcom.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Linux ARM Kernel <linux-arm-kernel@lists.infradead.org>,
        Linux IOMMU <iommu@lists.linux-foundation.org>, kvm@vger.kernel.org,
        BCM Kernel Feedback <bcm-kernel-feedback-list@broadcom.com>
Subject: Re: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM
 SMMUv3 driver
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On Thu, 20 Jul 2017 12:17:12 +0100
Robin Murphy <robin.murphy@arm.com> wrote:

> On 20/07/17 10:10, Will Deacon wrote:
> > On Thu, Jul 20, 2017 at 09:32:00AM +0530, Anup Patel wrote:  
> >> On Wed, Jul 19, 2017 at 5:23 PM, Will Deacon <will.deacon@arm.com> wrote:  
> >>> There are two things here:
> >>>
> >>>   1. iommu_present() is pretty useless, because it applies to a "bus" which
> >>>      doesn't actually tell you what you need to know for things like the
> >>>      platform_bus, where some masters might be upstream of an SMMU and
> >>>      others might not be.  
> >>
> >> I agree with you. The iommu_present() check in vfio_iommu_group_get()
> >> is not much useful. We only reach line which checks iommu_present()
> >> when iommu_group_get() returns NULL for given "struct device *". If there
> >> is no IOMMU group for a "struct device *" then it means there is no IOMMU
> >> HW doing translations for such device.
> >>
> >> If we drop the iommu_present() check (due to above reasons) in
> >> vfio_iommu_group_get() then we don't require the IOMMU_CAP_BYPASS
> >> and we can happily drop PATCH1, PATCH2, and PATCH3.
> >>
> >> I will remove the iommu_present() check in vfio_iommu_group_get()
> >> because it is only comes into actions when VFIO_NOIOMMU is
> >> enabled. This will also help us drop PATCH1-to-PATCH3.  
> > 
> > I don't think that's the right answer. Whilst iommu_present has obvious
> > shortcomings, its intention is clear: it should tell you whether a given
> > *device* is upstream of an IOMMU. So the right fix is to make this
> > per-device, instead of per-bus. Removing it altogether is worse than leaving
> > it like it is.  
> 
> Not really - if there is an IOMMU up and running to the point of setting
> bus ops, every device it cares about can be expected to have a group
> already (there are only a couple of drivers left that don't use groups,
> and they're hardly relevant to VFIO). Thus iommu_group_get() already is
> the de-facto per-device IOMMU check.
> 
> And having looked into it, I'm now spinning a couple of patches to
> finish off making groups truly mandatory so that that can be less
> de-facto ;)

No, look at vfio-noiommu and even vfio-mdev devices for devices which
have an iommu group but there is no physical iommu supporting them.
iommu_present() is how we can distinguish these groups and therefore
not generate a segfault in trying to use the full IOMMU API on them.
Thanks,

Alex