linux-kernel - Re: [PATCH v3 6/8] iommu/arm-smmu-v3: Move CD table to arm_smmu

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Date:   Wed, 2 Aug 2023 19:19:12 +0800
From:   Michael Shavit <mshavit@...gle.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, robin.murphy@....com,
        will@...nel.org, jean-philippe@...aro.org, nicolinc@...dia.com
Subject: Re: [PATCH v3 6/8] iommu/arm-smmu-v3: Move CD table to arm_smmu_master

On Wed, Aug 2, 2023 at 7:53 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Wed, Aug 02, 2023 at 02:35:23AM +0800, Michael Shavit wrote:
> > @@ -2465,6 +2440,22 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
> >       if (smmu_domain->stage != ARM_SMMU_DOMAIN_BYPASS)
> >               master->ats_enabled = arm_smmu_ats_supported(master);
> >
> > +     if (smmu_domain->stage == ARM_SMMU_DOMAIN_S1) {
> > +             if (!master->cd_table.cdtab) {
> > +                     ret = arm_smmu_alloc_cd_tables(master);
> > +                     if (ret) {
>
> Again, I didn't look very closely at your locking, but what lock is
> being held to protect the read of master->cd_table.cdtab ?

The cd_table is only written into (with write_ctx_desc) when something
attaches or detaches (SVA is a little weird, but it handles locking
internally, and blocks all non-sva attach/detach calls while enabled).
The cd_table itself is allocated on first attach, and freed on release.

Doesn't the iommu framework guarantee that attach_dev (and
release_device) won't have concurrent calls for a given master through
the group lock? I can add an internal lock if relying on the iommu
lock is not OK.

> > +                             master->domain = NULL;
> > +                             goto out_unlock;
>
> This is only the domain lock:
>         mutex_unlock(&smmu_domain->init_mutex);
>
> Which is no longer sufficient.

Hmmm yeah that lock looks misleading here. Let me move the unlock
further up so that it more clearly surrounds the section it protects.