linux-kernel - Re: [PATCH v4 03/12] iommu: Add attach/detach_dev

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Message-ID: <YmqqaXtqev9FUJo7@myrica>
Date:   Thu, 28 Apr 2022 15:53:29 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 03/12] iommu: Add attach/detach_dev_pasid domain ops

On Thu, Apr 21, 2022 at 01:21:12PM +0800, Lu Baolu wrote:
> Attaching an IOMMU domain to a PASID of a device is a generic operation
> for modern IOMMU drivers which support PASID-granular DMA address
> translation. Currently visible usage scenarios include (but not limited):
> 
>  - SVA (Shared Virtual Address)
>  - kernel DMA with PASID
>  - hardware-assist mediated device
> 
> This adds a pair of common domain ops for this purpose and adds helpers
> to attach/detach a domain to/from a {device, PASID}. 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, these
> interfaces only apply to devices belonging to the singleton groups, and
> the singleton is immutable in fabric i.e. not affected by hotplug.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
[...]
> +/*
> + * Use standard PCI bus topology, isolation features, and DMA
> + * alias quirks to check the immutable singleton attribute. If
> + * the device came from DT, assume it is static and then
> + * singleton can know from the device count in the group.
> + */
> +static bool device_group_immutable_singleton(struct device *dev)
> +{
> +	struct iommu_group *group = iommu_group_get(dev);
> +	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;
> +
> +	if (dev_is_pci(dev)) {
> +		struct pci_dev *pdev = to_pci_dev(dev);
> +
> +		/*
> +		 * The 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 (!pci_acs_path_enabled(pdev, NULL, REQ_ACS_FLAGS))
> +			return false;
> +
> +		/* Filter out devices which has any alias device. */
> +		if (pci_for_each_dma_alias(pdev, has_pci_alias, pdev))
> +			return false;

Aren't aliases already added to the group by pci_device_group()?  If so
it's part of the count check above

> +
> +		return true;
> +	}
> +
> +	/*
> +	 * If the device came from DT, assume it is static and then
> +	 * singleton can know from the device count in the group.
> +	 */
> +	return is_of_node(dev_fwnode(dev));

I don't think DT is relevant here because a platform device enumerated
through ACPI will also have its own group. It should be safe to stick to
what the IOMMU drivers declare with their device_group() callback. Except
for PCI those groups should be immutable so we can return true here.

Thanks,
Jean