linux-kernel - Re: [PATCH 01/11] iommu: Add device dma ownership set/release interfaces

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Date:   Tue, 16 Nov 2021 09:52:49 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     baolu.lu@...ux.intel.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        rafael@...nel.org, Diana Craciun <diana.craciun@....nxp.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/11] iommu: Add device dma ownership set/release
 interfaces

Hi Bjorn,

On 11/16/21 4:38 AM, Bjorn Helgaas wrote:
> On Mon, Nov 15, 2021 at 10:05:42AM +0800, Lu Baolu wrote:
>>  From the perspective of who is initiating the device to do DMA, device
>> DMA could be divided into the following types:
>>
>>          DMA_OWNER_KERNEL: kernel device driver intiates the DMA
>>          DMA_OWNER_USER: userspace device driver intiates the DMA
> 
> s/intiates/initiates/ (twice)

Yes.

> 
> As your first sentence suggests, the driver doesn't actually
> *initiate* the DMA in either case.  One of the drivers programs the
> device, and the *device* initiates the DMA.

You are right. I could rephrase it as:

"From the perspective of who is controlling the device to do DMA ..."

> 
>> DMA_OWNER_KERNEL and DMA_OWNER_USER are exclusive for all devices in
>> same iommu group as an iommu group is the smallest granularity of device
>> isolation and protection that the IOMMU subsystem can guarantee.
> 
> I think this basically says DMA_OWNER_KERNEL and DMA_OWNER_USER are
> attributes of the iommu_group (not an individual device), and it
> applies to all devices in the iommu_group.  Below, you allude to the
> fact that the interfaces are per-device.  It's not clear to me why you
> made a per-device interface instead of a per-group interface.

Yes, the attributes are of the iommu_group. We have both per-device and
per-iommu_group interfaces. The former is for device drivers and the
latter is only for vfio who has an iommu_group based iommu abstract.

>> This
>> extends the iommu core to enforce this exclusion when devices are
>> assigned to userspace.
>>
>> Basically two new interfaces are provided:
>>
>>          int iommu_device_set_dma_owner(struct device *dev,
>>                  enum iommu_dma_owner mode, struct file *user_file);
>>          void iommu_device_release_dma_owner(struct device *dev,
>>                  enum iommu_dma_owner mode);
>>
>> Although above interfaces are per-device, DMA owner is tracked per group
>> under the hood. An iommu group cannot have both DMA_OWNER_KERNEL
>> and DMA_OWNER_USER set at the same time. Violation of this assumption
>> fails iommu_device_set_dma_owner().
>>
>> Kernel driver which does DMA have DMA_OWNER_KENREL automatically
>> set/released in the driver binding process (see next patch).
> 
> s/DMA_OWNER_KENREL/DMA_OWNER_KERNEL/

Yes. Sorry for the typo.

> 
>> Kernel driver which doesn't do DMA should not set the owner type (via a
>> new suppress flag in next patch). Device bound to such driver is considered
>> same as a driver-less device which is compatible to all owner types.
>>
>> Userspace driver framework (e.g. vfio) should set DMA_OWNER_USER for
>> a device before the userspace is allowed to access it, plus a fd pointer to
>> mark the user identity so a single group cannot be operated by multiple
>> users simultaneously. Vice versa, the owner type should be released after
>> the user access permission is withdrawn.

Best regards,
baolu