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To:     Jason Gunthorpe <jgg@nvidia.com>,
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From:   Lu Baolu <baolu.lu@linux.intel.com>
Subject: Re: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma
 interfaces
Message-ID: <9a314095-3db9-30fc-2ed9-4e46d385036d@linux.intel.com>
Date:   Tue, 28 Sep 2021 21:35:05 +0800
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Hi Jason,

On 2021/9/28 19:57, Jason Gunthorpe wrote:
> On Tue, Sep 28, 2021 at 07:30:41AM +0000, Tian, Kevin wrote:
> 
>>> Also, don't call it "hint", there is nothing hinty about this, it has
>>> definitive functional impacts.
>>
>> possibly dma_mode (too broad?) or dma_usage
> 
> You just need a flag to specify if the driver manages DMA ownership
> itself, or if it requires the driver core to setup kernel ownership
> 
> DMA_OWNER_KERNEL
> DMA_OWNER_DRIVER_CONTROLLED
> 
> ?
> 
> There is a bool 'suprress_bind_attrs' already so it could be done like
> this:
> 
>   bool suppress_bind_attrs:1;
> 
>   /* If set the driver must call iommu_XX as the first action in probe() */
>   bool suppress_dma_owner:1;
> 
> Which is pretty low cost.

Yes. Pretty low cost to fix the BUG_ON() issue. Any kernel-DMA driver
binding is blocked if the device's iommu group has been put into user-
dma mode.

Another issue is, when putting a device into user-dma mode, all devices
belonging to the same iommu group shouldn't be bound with a kernel-dma
driver. Kevin's prototype checks this by READ_ONCE(dev->driver). This is
not lock safe as discussed below,

https://lore.kernel.org/linux-iommu/20210927130935.GZ964074@nvidia.com/

Any guidance on this?

Best regards,
baolu