linux-kernel - Re: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP

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Date:   Thu, 20 Jul 2017 10:10:03 +0100
From:   Will Deacon <will.deacon@....com>
To:     Anup Patel <anup.patel@...adcom.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Baptiste Reynal <b.reynal@...tualopensystems.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Scott Branden <sbranden@...adcom.com>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Linux ARM Kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>,
        kvm@...r.kernel.org,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>
Subject: Re: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM
 SMMUv3 driver

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@....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.

> >   2. If a master *is* upstream of an IOMMU and you want to use no-IOMMU,
> >      then the VFIO no-IOMMU code needs to be extended so that it creates
> >      an IDENTITY domain on that IOMMU.
> 
> The VFIO no-IOMMU mode is equivalent to Linux UIO hence having
> IDENTITY domain for VFIO no-IOMMU is not appropriate here.

Can you elaborate on this please? I don't understand the argument you're
making. It's like saying "I don't like eggs, therefore I don't drive a
car".

Will