linux-kernel - Re: [PATCH 1/5] iommu: Add capability IOMMU_CAP

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Date:   Wed, 19 Jul 2017 12:29:14 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Anup Patel <anup.patel@...adcom.com>,
        Will Deacon <will.deacon@....com>
Cc:     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 1/5] iommu: Add capability IOMMU_CAP_BYPASS

On 19/07/17 12:26, Anup Patel wrote:
> On Wed, Jul 19, 2017 at 4:53 PM, Will Deacon <will.deacon@....com> wrote:
>> On Wed, Jul 19, 2017 at 04:49:00PM +0530, Anup Patel wrote:
>>> On Wed, Jul 19, 2017 at 4:28 PM, Robin Murphy <robin.murphy@....com> wrote:
>>>> On 19/07/17 10:33, Anup Patel wrote:
>>>>> Some of the IOMMUs (such as ARM SMMU) are capable of bypassing
>>>>> transactions for which no IOMMU domain is configured.
>>>>>
>>>>> This patch adds IOMMU_CAP_BYPASS which can be used by IOMMU
>>>>> drivers to advertise transation bypass capability of an IOMMU.
>>>>
>>>> Whatever the intended semantics of this are, I can't help thinking it
>>>> would be better served by allowing callers to explicitly allocate their
>>>> own IOMMU_DOMAIN_IDENTITY domains. That would also be useful for the
>>>> problem we have with legacy virtio devices behind real IOMMUs.
>>>
>>> We want to use VFIO no-IOMMU mode for FlexRM device but
>>> currently it does not allow on our SOC because IOMMU ops are
>>> registered for platform bus.
>>
>> Why do you want to use no-IOMMU mode if you have an IOMMU, and why you do
>> think the individual IOMMU drivers are the place to implement this?
>>
>> NAK to the SMMU patches, for the reasons outlined by Robin.
> 
> We have limited number of SMRs on our SOC.
> 
> There are lot of devices for which we can potentially
> configure SMMU but then due to limited number of
> SMRs so we use SMMU only for certain devices.

Is the stream ID allocation so whacked out that you can't use masking?

Robin.
> For FlexRM device on our SOC, we don't intend to
> use SMMU hence we need VFIO no-IOMMU mode
> working for FlexRM device on our SOC.
> 
> Please re-consider your NAK.
> 
> Regards,
> Anup
>