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From: anup.patel@broadcom.com (Anup Patel)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM SMMUv3 driver
Date: Thu, 20 Jul 2017 16:38:09 +0530	[thread overview]
Message-ID: <CAALAos8571-UVp7Ky81qFESVcNcWx_H2VOemFt_++NFCxcMg3w@mail.gmail.com> (raw)
In-Reply-To: <20170720091003.GA17837@arm.com>

On Thu, Jul 20, 2017 at 2:40 PM, Will Deacon <will.deacon@arm.com> 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.
>
>> >   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".
>

Like I said, VFIO no-IOMMU mode for a device means device transactions
will not go through any IOMMU. That's why having IDENTITY domain for
device using VFIO no-IOMMU is not semantically correct. The analogy you
proposed does not apply here.

Anyways, this patch has nothing to do with FlexRM support for
VFIO platform hence I will drop it. Fixing VFIO no-IOMMU mode
can be a separate patchset.

Regards,
Anup

  reply	other threads:[~2017-07-20 11:08 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-07-19  9:33 [PATCH 0/5] FlexRM support in VFIO platform Anup Patel
2017-07-19  9:33 ` [PATCH 1/5] iommu: Add capability IOMMU_CAP_BYPASS Anup Patel
2017-07-19 10:58   ` Robin Murphy
2017-07-19 11:19     ` Anup Patel
2017-07-19 11:23       ` Will Deacon
2017-07-19 11:26         ` Anup Patel
2017-07-19 11:29           ` Robin Murphy
2017-07-19 11:30           ` Will Deacon
2017-07-19 11:33             ` Anup Patel
2017-07-19  9:33 ` [PATCH 2/5] iommu/arm-smmu: add IOMMU_CAP_BYPASS to the ARM SMMU driver Anup Patel
2017-07-19 10:59   ` Robin Murphy
2017-07-19  9:33 ` [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM SMMUv3 driver Anup Patel
2017-07-19 11:00   ` Robin Murphy
2017-07-19 11:23     ` Anup Patel
2017-07-19 11:25       ` Will Deacon
2017-07-19 11:31         ` Anup Patel
2017-07-19 11:33           ` Will Deacon
2017-07-19 11:39             ` Anup Patel
2017-07-19 11:53               ` Will Deacon
2017-07-20  4:02                 ` Anup Patel
2017-07-20  9:10                   ` Will Deacon
2017-07-20 11:08                     ` Anup Patel [this message]
2017-07-20 11:17                       ` Will Deacon
2017-07-20 11:17                     ` Robin Murphy
2017-07-24 17:16                       ` Alex Williamson
2017-07-24 17:23                         ` Robin Murphy
2017-07-24 19:06                           ` Alex Williamson
2017-07-25  8:59                             ` Anup Patel
2017-07-19  9:33 ` [PATCH 4/5] vfio: Allow No-IOMMU mode for IOMMUs with bypass capability Anup Patel
2017-07-19  9:33 ` [PATCH 5/5] vfio: platform: reset: Add Broadcom FlexRM reset module Anup Patel
2017-07-19 16:50   ` Scott Branden
2017-07-20  4:05     ` Anup Patel
2017-07-19 10:57 ` [PATCH 0/5] FlexRM support in VFIO platform Robin Murphy
2017-07-19 11:17   ` Anup Patel
2017-07-19 11:25     ` Robin Murphy
2017-07-19 11:28       ` Anup Patel

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