Re: [PATCH v7 03/10] iommu/sva: Add iommu_sva_domain support

Baolu Lu
2022-05-25T04:50:18Z

On 2022/5/24 17:39, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@linux.intel.com>
>> Sent: Thursday, May 19, 2022 3:21 PM
>>
>> The iommu_sva_domain represents a hardware pagetable that the IOMMU
>> hardware could use for SVA translation. This adds some infrastructure
>> to support SVA domain in the iommu common layer. It includes:
>>
>> - Add a new struct iommu_sva_domain and new IOMMU_DOMAIN_SVA
>> domain
>> type.
>> - Add a new domain ops pointer in iommu_ops. The IOMMU drivers that
>> support SVA should provide the callbacks.
>> - Add helpers to allocate and free an SVA domain.
>> - Add helpers to set an SVA domain to a device and the reverse
>> operation.
>>
>> Some buses, like PCI, route packets without considering the PASID value.
>> Thus a DMA target address with PASID might be treated as P2P if the
>> address falls into the MMIO BAR of other devices in the group. To make
>> things simple, the attach/detach interfaces only apply to devices
>> belonging to the singleton groups, and the singleton is immutable in
>> fabric i.e. not affected by hotplug.
>>
>> The iommu_set/block_device_pasid() can be used for other purposes,
>> such as kernel DMA with pasid, mediation device, etc. Hence, it is put
>> in the iommu.c.
>
> usually we have 'set/clear' pair or 'allow/block'. Having 'set' paired
> with 'block' doesn't read very clearly.

Yes. Let's still use the attach/detach semantics.

>
>> +static bool device_group_immutable_singleton(struct device *dev)
>> +{
>> + struct iommu_group *group = iommu_group_get(dev);
>
> what about passing group as the parameter since the caller will
> get the group again right after calling this function? In that case
> the function could be renamed as:
>
> iommu_group_immutable_singleton()
>
> or be shorter:
>
> iommu_group_fixed_singleton()

Fair enough. I will tune it as below:

+static bool iommu_group_immutable_singleton(struct iommu_group *group)
+{
+ int count;
+
+ mutex_lock(&group->mutex);
+ count = iommu_group_device_count(group);
+ mutex_unlock(&group->mutex);
+
+ if (count != 1)
+ return false;
+
+ /*
+ * The PCI device could be considered to be fully isolated if all
+ * devices on the path from the device to the host-PCI bridge are
+ * protected from peer-to-peer DMA by ACS.
+ */
+ if (dev_is_pci(dev))
+ return pci_acs_path_enabled(to_pci_dev(dev), NULL,
+ REQ_ACS_FLAGS);
+
+ /*
+ * Otherwise, the device came from DT/ACPI, assume it is static and
+ * then singleton can know from the device count in the group.
+ */
+ return true;
+}

>
>> + int count;
>> +
>> + if (!group)
>> + return false;
>> +
>> + mutex_lock(&group->mutex);
>> + count = iommu_group_device_count(group);
>> + mutex_unlock(&group->mutex);
>> + iommu_group_put(group);
>> +
>> + if (count != 1)
>> + return false;
>
> For non-pci devices above doesn't check anything against immutable.
> Please add some comment to explain why doing so is correct.

Yes, as above code shows.

>
>> +
>> + /*
>> + * The PCI device could be considered to be fully isolated if all
>> + * devices on the path from the device to the host-PCI bridge are
>> + * protected from peer-to-peer DMA by ACS.
>> + */
>> + if (dev_is_pci(dev))
>> + return pci_acs_path_enabled(to_pci_dev(dev), NULL,
>> + REQ_ACS_FLAGS);
>> +
>> + return true;
>> +}
>> +

Best regards,
baolu