Re: [PATCH 6/8] iommu/arm-smmu-v3: Support auxiliary domains

Jean-Philippe Brucker
2019-09-19T15:06:39Z

On Wed, Jun 26, 2019 at 06:59:59PM +0100, Will Deacon wrote:
> > @@ -666,8 +668,14 @@ struct arm_smmu_domain {
> >
> > struct iommu_domain domain;
> >
> > + /* Unused in aux domains */
> > struct list_head devices;
> > spinlock_t devices_lock;
> > +
> > + /* Auxiliary domain stuff */
> > + struct arm_smmu_domain *parent;
> > + ioasid_t ssid;
> > + unsigned long aux_nr_devs;
>
> Maybe use a union to avoid comments about what is used/unused?

OK

> > +static void arm_smmu_aux_detach_dev(struct iommu_domain *domain, struct device *dev)
> > +{
> > + struct iommu_domain *parent_domain;
> > + struct arm_smmu_domain *parent_smmu_domain;
> > + struct arm_smmu_master *master = dev_to_master(dev);
> > + struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
> > +
> > + if (!arm_smmu_dev_feature_enabled(dev, IOMMU_DEV_FEAT_AUX))
> > + return;
> > +
> > + parent_domain = iommu_get_domain_for_dev(dev);
> > + if (!parent_domain)
> > + return;
> > + parent_smmu_domain = to_smmu_domain(parent_domain);
> > +
> > + mutex_lock(&smmu_domain->init_mutex);
> > + if (!smmu_domain->aux_nr_devs)
> > + goto out_unlock;
> > +
> > + if (!--smmu_domain->aux_nr_devs) {
> > + arm_smmu_write_ctx_desc(parent_smmu_domain, smmu_domain->ssid,
> > + NULL);
> > + /*
> > + * TLB doesn't need invalidation since accesses from the device
> > + * can't use this domain's ASID once the CD is clear.
> > + *
> > + * Sadly that doesn't apply to ATCs, which are PASID tagged.
> > + * Invalidate all other devices as well, because even though
> > + * they weren't 'officially' attached to the auxiliary domain,
> > + * they could have formed ATC entries.
> > + */
> > + arm_smmu_atc_inv_domain(smmu_domain, 0, 0);
>
> I've been struggling to understand the locking here, since both
> arm_smmu_write_ctx_desc and arm_smmu_atc_inv_domain take and release the
> devices_lock for the domain. Is there not a problem with devices coming and
> going in-between the two calls?

Yes, I need to think about this more. I bet there are plenty more issues
like this. For example I don't think I currently prevent the parent
domain from disappearing while auxiliary domains are attached.

> > static struct iommu_ops arm_smmu_ops = {
> > .capable = arm_smmu_capable,
> > .domain_alloc = arm_smmu_domain_alloc,
> > @@ -2539,6 +2772,13 @@ static struct iommu_ops arm_smmu_ops = {
> > .of_xlate = arm_smmu_of_xlate,
> > .get_resv_regions = arm_smmu_get_resv_regions,
> > .put_resv_regions = arm_smmu_put_resv_regions,
> > + .dev_has_feat = arm_smmu_dev_has_feature,
> > + .dev_feat_enabled = arm_smmu_dev_feature_enabled,
> > + .dev_enable_feat = arm_smmu_dev_enable_feature,
> > + .dev_disable_feat = arm_smmu_dev_disable_feature,
>
> Why can't we use the existing ->capable and ->dev_{get,set}_attr callbacks
> for this?

->capable isn't very useful because it applies to all SMMUs in the
system. The existing ->{get,set}_attr callbacks apply to an
iommu_domain. I think the main reason for doing it on endpoints was that
it would be tedious to keep track of capabilities when attaching and
detaching devices to a domain, especially for drivers that allow
multiple IOMMUs per domain [1]. There were more discussions, and in the
end we agreed on this API for device attributes [2].

Thanks,
Jean

[1] https://lore.kernel.org/lkml/aa1ff748-c2ec-acc0-f1d9-cdff2b131e58@linux.intel.com/
[2] https://lore.kernel.org/linux-iommu/20181207102926.GM16835@8bytes.org/