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Subject: Re: [PATCH] vhost: introduce vDPA based backend
To:     Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Gunthorpe <jgg@mellanox.com>
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        "rob.miller@broadcom.com" <rob.miller@broadcom.com>,
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        Parav Pandit <parav@mellanox.com>,
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From:   Zhu Lingshan <lingshan.zhu@linux.intel.com>
Message-ID: <4b1753eb-b281-17e0-6636-849ac20cbe50@linux.intel.com>
Date:   Thu, 6 Feb 2020 11:21:27 +0800
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On 2/6/2020 11:11 AM, Jason Wang wrote:
>
> On 2020/2/5 下午9:14, Michael S. Tsirkin wrote:
>> On Wed, Feb 05, 2020 at 08:56:48AM -0400, Jason Gunthorpe wrote:
>>> On Wed, Feb 05, 2020 at 03:50:14PM +0800, Jason Wang wrote:
>>>>> Would it be better for the map/umnap logic to happen inside each 
>>>>> device ?
>>>>> Devices that needs the IOMMU will call iommu APIs from inside the 
>>>>> driver callback.
>>>> Technically, this can work. But if it can be done by vhost-vpda it 
>>>> will make
>>>> the vDPA driver more compact and easier to be implemented.
>>> Generally speaking, in the kernel, it is normal to not hoist code of
>>> out drivers into subsystems until 2-3 drivers are duplicating that
>>> code. It helps ensure the right design is used
>>>
>>> Jason
>> That's up to the sybsystem maintainer really, as there's also some
>> intuition involved in guessing a specific API is widely useful.
>> In-kernel APIs are flexible, if we find something isn't needed we just
>> drop it.
>>
>
> If I understand correctly. At least Intel (Ling Shan) and Brodcom 
> (Rob) doesn't want to deal with DMA stuffs in their driver.
>
> Anyway since the DMA bus operations is optional, driver may still 
> choose to do DMA by itself if they want even if it requires platform 
> IOMMU to work.
>
> Thanks
>
Many Thanks if this could be done. The parent device has DMA 
capabilities and dma ops implemented, we hope can make use of it, as 
discussed in the vdpa thread.