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Date:   Fri, 9 Jun 2023 09:44:10 +0800
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Cc:     baolu.lu@linux.intel.com, Michael Shavit <mshavit@google.com>,
        Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Joerg Roedel <joro@8bytes.org>, jean-philippe@linaro.org,
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        iommu@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 14/18] iommu/arm-smmu-v3: Support domains with shared
 CDs
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To:     Jason Gunthorpe <jgg@nvidia.com>
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From:   Baolu Lu <baolu.lu@linux.intel.com>
In-Reply-To: <ZIHaEFJFVQqvJvmU@nvidia.com>
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On 6/8/23 9:39 PM, Jason Gunthorpe wrote:
> On Thu, Jun 08, 2023 at 10:39:23AM +0800, Baolu Lu wrote:
>> On 6/7/23 7:59 PM, Jason Gunthorpe wrote:
>>> On Wed, Jun 07, 2023 at 12:06:07AM +0530, Michael Shavit wrote:
>>>>> What we definately shouldn't do is try to have different SVA
>>>>> iommu_domain's pointing at the same ASID. That is again making SVA
>>>>> special, which we are trying to get away from ????
>>>> Fwiw, this change is preserving the status-quo in that regard;
>>>> arm-smmu-v3-sva.c is already doing this. But yes, I agree that
>>>> resolving the limitation is a better long term solution... and
>>>> something I can try to look at further.
>>> I suppose we also don't really have a entirely clear picture what
>>> allocating multiple SVA domains should even do in the iommu driver.
>>>
>>> The driver would like to share the ASID, but things are much cleaner
>>> for everything if the driver model has ASID 1:1 with the iommu_domain.
>> This means that each ASID should be mapped to a single IOMMU domain.
>> This is conceptually right as iommu_domain represents a hardware page
>> table. For SVA, it's an mm_struct.
>>
>> So in my mind, each sva_domain should have a 1:1 relationship with an
>> mm_struct.
> If we want to support multiple iommu drivers then we should support
> multiple iommu_domains per mm_struct so that each driver can have its
> own. In this world if each instance wants its own iommu_domain it is
> not a big deal.
> 
> Drivers that can share iommu_domains across instances should probably
> also share sva iommu_domains across instances.
> 
> Most real systems have only one iommu driver and we'd like the good
> iommu drivers to be able to share domains across instances, so we'd
> expect only 1 iommu_domain per mm struct.

Yes. You are right. I overlooked the multiple-drivers case. So we stay
on the same page now.

Best regards,
baolu