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

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Date:   Wed, 7 Jun 2023 00:06:07 +0530
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 Tue, Jun 6, 2023 at 10:39 PM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Tue, Jun 06, 2023 at 08:07:50PM +0800, Michael Shavit wrote:
> > SVA may attach a CD to masters that have different upstream SMMU
> > devices. The arm_smmu_domain structure can only be attached to a single
> > upstream SMMU device however.
>
> Isn't that pretty much because we don't support replicating
> invalidations to each of the different SMMU instances?
....
> I would expect different SMMU devices to be handled by allowing single
> domains to be attached to a list of masters where the masters can all
> be from different instances.

Oh you're right! When I first looked at this, arm_smmu_domain still
held an arm_smmu_s1_cfg which depends on the SMMU device. But
attaching a single (stage 1) arm_smmu_domain to multiple SMMUs looks
much more tractable after the first half of this patch series. We can
add a handle to the smmu device in the new arm_smmu_attached_domain
struct for this purpose.

> 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.

> You might try to stop your series here and get the first batch of
> patches done. This latter bit seems to be a seperate topic?

The main motivation for this part of the series is to reduce
inconsistencies with the smmu_domain->attached_domains list and
arm-smmu-v3 functions that rely on that list. Specifically to reach
the last patch in the series: "iommu/arm-smmu-v3-sva: Remove
atc_inv_domain_ssid".

Splitting this part into a follow-up patch series would definitely be
easier and helpful if you're all ok with it :) .