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Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus
To:     "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Gunthorpe <jgg@mellanox.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <26fa6211-6625-6dc6-f5df-7a124d8c53ae@redhat.com>
Date:   Wed, 22 Jan 2020 14:18:01 +0800
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On 2020/1/21 =E4=B8=8B=E5=8D=8810:17, Michael S. Tsirkin wrote:
> On Tue, Jan 21, 2020 at 02:05:04PM +0000, Jason Gunthorpe wrote:
>> On Tue, Jan 21, 2020 at 03:15:43AM -0500, Michael S. Tsirkin wrote:
>>>> This sounds more flexible e.g driver may choose to implement static =
mapping
>>>> one through commit. But a question here, it looks to me this still r=
equires
>>>> the DMA to be synced with at least commit here. Otherwise device may=
 get DMA
>>>> fault? Or device is expected to be paused DMA during begin?
>>>>
>>>> Thanks
>>> For example, commit might switch one set of tables for another,
>>> without need to pause DMA.
>> I'm not aware of any hardware that can do something like this
>> completely atomically..
> FWIW VTD can do this atomically.
>
>> Any mapping change API has to be based around add/remove regions
>> without any active DMA (ie active DMA is a guest error the guest can
>> be crashed if it does this)
>>
>> Jason
> Right, lots of cases are well served by only changing parts of
> mapping that aren't in active use. Memory hotplug is such a case.
> That's not the same as a completely static mapping.


For hotplug it should be fine with current Qemu since it belongs to=20
different memory regions. So each dimm should have its own dedicated map=20
entries in IOMMU.

But I'm not sure if the merging logic in current vhost memory listener=20
may cause any trouble, we may need to disable it.

Thanks


>