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Subject: Re: [PATCH] vhost: introduce vDPA based backend
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Shahaf Shuler <shahafs@mellanox.com>,
        Tiwei Bie <tiwei.bie@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        Jason Gunthorpe <jgg@mellanox.com>,
        "rob.miller@broadcom.com" <rob.miller@broadcom.com>,
        "haotian.wang@sifive.com" <haotian.wang@sifive.com>,
        "eperezma@redhat.com" <eperezma@redhat.com>,
        "lulu@redhat.com" <lulu@redhat.com>,
        Parav Pandit <parav@mellanox.com>,
        "rdunlap@infradead.org" <rdunlap@infradead.org>,
        "hch@infradead.org" <hch@infradead.org>,
        Jiri Pirko <jiri@mellanox.com>,
        "hanand@xilinx.com" <hanand@xilinx.com>,
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        "maxime.coquelin@redhat.com" <maxime.coquelin@redhat.com>,
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        "dan.daly@intel.com" <dan.daly@intel.com>,
        "cunming.liang@intel.com" <cunming.liang@intel.com>,
        "zhihong.wang@intel.com" <zhihong.wang@intel.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <7519cde6-2c79-6867-d72d-05be73d947a6@redhat.com>
Date:   Thu, 6 Feb 2020 11:07:55 +0800
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On 2020/2/5 =E4=B8=8B=E5=8D=885:23, Michael S. Tsirkin wrote:
> On Wed, Feb 05, 2020 at 03:50:14PM +0800, Jason Wang wrote:
>> On 2020/2/5 =C3=A4=C2=B8=E2=80=B9=C3=A5=C2=8D=CB=863:15, Shahaf Shuler=
 wrote:
>>> Wednesday, February 5, 2020 4:03 AM, Tiwei Bie:
>>>> Subject: Re: [PATCH] vhost: introduce vDPA based backend
>>>>
>>>> On Tue, Feb 04, 2020 at 11:30:11AM +0800, Jason Wang wrote:
>>>>> On 2020/1/31 =C3=A4=C2=B8=C5=A0=C3=A5=C2=8D=CB=8611:36, Tiwei Bie w=
rote:
>>>>>> This patch introduces a vDPA based vhost backend. This backend is
>>>>>> built on top of the same interface defined in virtio-vDPA and
>>>>>> provides a generic vhost interface for userspace to accelerate the
>>>>>> virtio devices in guest.
>>>>>>
>>>>>> This backend is implemented as a vDPA device driver on top of the
>>>>>> same ops used in virtio-vDPA. It will create char device entry nam=
ed
>>>>>> vhost-vdpa/$vdpa_device_index for userspace to use. Userspace can
>>>>>> use vhost ioctls on top of this char device to setup the backend.
>>>>>>
>>>>>> Signed-off-by: Tiwei Bie<tiwei.bie@intel.com>
>>> [...]
>>>
>>>>>> +static long vhost_vdpa_do_dma_mapping(struct vhost_vdpa *v) {
>>>>>> +	/* TODO: fix this */
>>>>> Before trying to do this it looks to me we need the following durin=
g
>>>>> the probe
>>>>>
>>>>> 1) if set_map() is not supported by the vDPA device probe the IOMMU
>>>>> that is supported by the vDPA device
>>>>> 2) allocate IOMMU domain
>>>>>
>>>>> And then:
>>>>>
>>>>> 3) pin pages through GUP and do proper accounting
>>>>> 4) store GPA->HPA mapping in the umem
>>>>> 5) generate diffs of memory table and using IOMMU API to setup the =
dma
>>>>> mapping in this method
>>>>>
>>>>> For 1), I'm not sure parent is sufficient for to doing this or need=
 to
>>>>> introduce new API like iommu_device in mdev.
>>>> Agree. We may also need to introduce something like the iommu_device=
.
>>>>
>>> Would it be better for the map/umnap logic to happen inside each devi=
ce ?
>>> Devices that needs the IOMMU will call iommu APIs from inside the dri=
ver callback.
>> Technically, this can work. But if it can be done by vhost-vpda it wil=
l make
>> the vDPA driver more compact and easier to be implemented.
>>
>>
>>> Devices that has other ways to do the DMA mapping will call the propr=
ietary APIs.
>> To confirm, do you prefer:
>>
>> 1) map/unmap
>>
>> or
>>
>> 2) pass all maps at one time?
>>
>> Thanks
>>
>>
> I mean we really already have both right? ATM 1 is used with an iommu
> and 2 without. I guess we can also have drivers ask for either or both
> ...


Yes, that looks better.

Thanks