linux-kernel - Re: [PATCH v7 03/10] iommu/sva: Add iommu_sva

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Message-ID: <caae0926-b8f8-a64d-5c33-441e33d0c1dc@linux.intel.com>
Date:   Wed, 25 May 2022 12:50:04 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Jiang, Dave" <dave.jiang@...el.com>, Vinod Koul <vkoul@...nel.org>
Cc:     baolu.lu@...ux.intel.com, Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH v7 03/10] iommu/sva: Add iommu_sva_domain support

On 2022/5/24 17:39, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.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