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SubjectRe: [PATCH 6/8] iommu/arm-smmu-v3: Support auxiliary domains
Date
On 26/06/2019 18:59, Will Deacon wrote:
>> +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 it's a problem. I suppose we could take the parent's init_mutex
(making sure that it protects detach_dev() as well.

First I need to figure out how to prevent the parent domain from
disappearing when auxiliary domains are attached, I seem to have forgotten
that. I think checking if AUXD is enabled in the device passed to
attach_dev() should be sufficient - that's what I do for SVA. But the
IOMMU API isn't quite ready to handle failure in iommu_detach_device() at
the moment. VFIO will free the domain even if it's still attached.

>
>> + } else {
>> + struct arm_smmu_cmdq_ent cmd;
>> +
>> + /* Invalidate only this device's ATC */
>> + if (master->ats_enabled) {
>> + arm_smmu_atc_inv_to_cmd(smmu_domain->ssid, 0, 0, &cmd);
>> + arm_smmu_atc_inv_master(master, &cmd);
>> + }
>> + }
>> +out_unlock:
>> + mutex_unlock(&smmu_domain->init_mutex);
>> +}
>> +
>> +static int arm_smmu_aux_get_pasid(struct iommu_domain *domain, struct device *dev)
>> +{
>> + struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
>> +
>> + return smmu_domain->ssid ?: -EINVAL;
>> +}
>> +
>> 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. 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 Joerg proposed the current API for device attributes [2]

[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/

Thanks,
Jean

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