linux-kernel - Re: [PATCH v2 14/18] iommu/arm-smmu-v3: Support domains with shared CDs

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Message-ID: <CAKHBV25g2M-35ZQ9X-mx-La7jVDgBm6rw-umOkNdgu3_jAiA-A@mail.gmail.com>
Date:   Wed, 14 Jun 2023 21:30:34 +0800
From:   Michael Shavit <mshavit@...gle.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>, jean-philippe@...aro.org,
        nicolinc@...dia.com, baolu.lu@...ux.intel.com,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 14/18] iommu/arm-smmu-v3: Support domains with shared CDs

On Wed, Jun 14, 2023 at 5:57 PM Robin Murphy <robin.murphy@....com> wrote:
>
> In practice it would be entirely reasonable to only support
> cross-instance attach between instances with matching capabilities such
> that they *can* share the pagetable directly.

On Wed, Jun 14, 2023 at 8:10 PM Jason Gunthorpe <jgg@...dia.com> wrote:
> When we talk about multi instance support, it means the iommu_domain -
> in whatever fixed IO page table format it uses - can be attached to
> any SMMU instance that supports it as a compatible page table format.
>
> ARM doesn't quite reach this model, but once it passes the finalize
> step it does. The goal is to move finalize to allocate. So for your
> purposes you can ignore the difference.

Got you. Failing the atach when the page table format is incompatible
with the smmu device is a lot easier to handle. I didn't notice that
SVA was already checking this elsewhere.
I can give the multi-instance support a try (with the rest of these
patches on top) and send it out as a follow-up series to this one.

> I think it is good to make progress, it looked to me like the first
> part stood alone fairly well and was an improvement on its own.

Sorry for the noobie question; it's not 100% obvious to me what the
next step is here. Is there anything I should do to progress that
first part forward?