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Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus
To:     Shahaf Shuler <shahafs@mellanox.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Jason Gunthorpe <jgg@mellanox.com>,
        Rob Miller <rob.miller@broadcom.com>,
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        "Liang, Cunming" <cunming.liang@intel.com>,
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <ef3ca685-2cbe-ce7c-c9c0-cbb5caa08db2@redhat.com>
Date:   Tue, 21 Jan 2020 15:57:29 +0800
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On 2020/1/21 =E4=B8=8B=E5=8D=882:01, Shahaf Shuler wrote:
> Tuesday, January 21, 2020 12:00 AM, Michael S. Tsirkin:
>> Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus
>>
>> On Mon, Jan 20, 2020 at 09:47:18PM +0000, Shahaf Shuler wrote:
>>> Monday, January 20, 2020 11:25 PM, Michael S. Tsirkin:
>>>> Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus
>>>>
>>>> On Mon, Jan 20, 2020 at 08:51:43PM +0000, Shahaf Shuler wrote:
>>>>> Monday, January 20, 2020 7:50 PM, Jason Gunthorpe:
>>>>>> Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus
>>>>>>
>>>>>> On Mon, Jan 20, 2020 at 04:43:53PM +0800, Jason Wang wrote:
>>>>>>> This is similar to the design of platform IOMMU part of
>>>>>>> vhost-vdpa. We decide to send diffs to platform IOMMU there.
>>>>>>> If it's ok to do that in driver, we can replace set_map with
>>>>>>> incremental API
>>>> like map()/unmap().
>>>>>>> Then driver need to maintain rbtree itself.
>>>>>> I think we really need to see two modes, one where there is a
>>>>>> fixed translation without dynamic vIOMMU driven changes and one
>>>>>> that supports vIOMMU.
>>>>>>
>>>>>> There are different optimization goals in the drivers for these
>>>>>> two configurations.
>>>>> +1.
>>>>> It will be best to have one API for static config (i.e. mapping
>>>>> can be set only before virtio device gets active), and one API for
>>>>> dynamic changes that can be set after the virtio device is active.
>>>> Frankly I don't see when we'd use the static one.
>>>> Memory hotplug is enabled for most guests...
>>> The fact memory hotplug is enabled doesn't necessarily means there is=
 not
>> cold-plugged memory on the hot plugged slots.
>>> So your claim is majority of guests are deployed w/o any cold-plugged
>> memory?
>>
>> Sorry for not being clear. I was merely saying that dynamic one can't =
be
>> optional, and static one can. So how about we start just with the dyna=
mic
>> one, then add the static one as a later optimization?
> Since we have the use case (cold plugged memory to guest, e.g. when pop=
ulated w/ hugepages) I think we should start w/ both. The static one can =
be optional for drivers.
>
> Moreover am not yet clear about the suggested API for dynamic, can you =
share the prototype you have in mind?
> Also will it be :
> 1. multiple add_map and then flag the driver to set
> Or
> 2. each add_map should be set by the driver as stand alone.


For dynamic one, it looks to me that introducing add_map()/del_map() bus=20
operations is much more cleaner than reusing current set_map() one.

Thanks


>