Re: [PATCH v8 7/9] vfio/mdev: Add iommu related member in mdev_device

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi Jason,

On 4/7/21 4:00 AM, Jason Gunthorpe wrote:
> On Mon, Mar 25, 2019 at 09:30:34AM +0800, Lu Baolu wrote:
>> A parent device might create different types of mediated
>> devices. For example, a mediated device could be created
>> by the parent device with full isolation and protection
>> provided by the IOMMU. One usage case could be found on
>> Intel platforms where a mediated device is an assignable
>> subset of a PCI, the DMA requests on behalf of it are all
>> tagged with a PASID. Since IOMMU supports PASID-granular
>> translations (scalable mode in VT-d 3.0), this mediated
>> device could be individually protected and isolated by an
>> IOMMU.
>>
>> This patch adds a new member in the struct mdev_device to
>> indicate that the mediated device represented by mdev could
>> be isolated and protected by attaching a domain to a device
>> represented by mdev->iommu_device. It also adds a helper to
>> add or set the iommu device.
>>
>> * mdev_device->iommu_device
>>    - This, if set, indicates that the mediated device could
>>      be fully isolated and protected by IOMMU via attaching
>>      an iommu domain to this device. If empty, it indicates
>>      using vendor defined isolation, hence bypass IOMMU.
>>
>> * mdev_set/get_iommu_device(dev, iommu_device)
>>    - Set or get the iommu device which represents this mdev
>>      in IOMMU's device scope. Drivers don't need to set the
>>      iommu device if it uses vendor defined isolation.
>>
>> Cc: Ashok Raj <ashok.raj@intel.com>
>> Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
>> Cc: Kevin Tian <kevin.tian@intel.com>
>> Cc: Liu Yi L <yi.l.liu@intel.com>
>> Suggested-by: Kevin Tian <kevin.tian@intel.com>
>> Suggested-by: Alex Williamson <alex.williamson@redhat.com>
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> Reviewed-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
>> ---
>>   drivers/vfio/mdev/mdev_core.c    | 18 ++++++++++++++++++
>>   drivers/vfio/mdev/mdev_private.h |  1 +
>>   include/linux/mdev.h             | 14 ++++++++++++++
>>   3 files changed, 33 insertions(+)
>>
>> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
>> index b96fedc77ee5..1b6435529166 100644
>> +++ b/drivers/vfio/mdev/mdev_core.c
>> @@ -390,6 +390,24 @@ int mdev_device_remove(struct device *dev, bool force_remove)
>>   	return 0;
>>   }
>>   
>> +int mdev_set_iommu_device(struct device *dev, struct device *iommu_device)
>> +{
>> +	struct mdev_device *mdev = to_mdev_device(dev);
>> +
>> +	mdev->iommu_device = iommu_device;
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL(mdev_set_iommu_device);
> 
> I was looking at these functions when touching the mdev stuff and I
> have some concerns.
> 
> 1) Please don't merge dead code. It is a year later and there is still
>     no in-tree user for any of this. This is not our process. Even
>     worse it was exported so it looks like this dead code is supporting
>     out of tree modules.
> 
> 2) Why is this like this? Every struct device already has a connection
>     to the iommu layer and every mdev has a struct device all its own.
> 
>     Why did we need to add special 'if (mdev)' stuff all over the
>     place? This smells like the same abuse Thomas
>     and I pointed out for the interrupt domains.
> 
>     After my next series the mdev drivers will have direct access to
>     the vfio_device. So an alternative to using the struct device, or
>     adding 'if mdev' is to add an API to the vfio_device world to
>     inject what iommu configuration is needed from that direction
>     instead of trying to discover it from a struct device.

Just want to make sure that I understand you correctly.

We should use the existing IOMMU in-kernel APIs to connect mdev with the
iommu subsystem, so that the upper lays don't need to use something
like (if dev_is_mdev) to handle mdev differently. Do I get you
correctly?

> 
> 3) The vfio_bus_is_mdev() and related symbol_get() nonsense in
>     drivers/vfio/vfio_iommu_type1.c has to go, for the same reasons
>     it was not acceptable to do this for the interrupt side either.

Yes. Agreed. I will look into it.

> 
> 4) It seems pretty clear to me this will be heavily impacted by the
>     /dev/ioasid discussion. Please consider removing the dead code now.
> 
> Basically, please fix this before trying to get idxd mdev merged as
> the first user.
> 
> Jason
> 

Best regards,
baolu