linux-kernel - Re: [PATCH] vhost: introduce vDPA based backend

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Date:   Wed, 5 Feb 2020 15:50:14 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Shahaf Shuler <shahafs@...lanox.com>,
        Tiwei Bie <tiwei.bie@...el.com>
Cc:     "mst@...hat.com" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jason Gunthorpe <jgg@...lanox.com>,
        "rob.miller@...adcom.com" <rob.miller@...adcom.com>,
        "haotian.wang@...ive.com" <haotian.wang@...ive.com>,
        "eperezma@...hat.com" <eperezma@...hat.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        Parav Pandit <parav@...lanox.com>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "hch@...radead.org" <hch@...radead.org>,
        Jiri Pirko <jiri@...lanox.com>,
        "hanand@...inx.com" <hanand@...inx.com>,
        "mhabets@...arflare.com" <mhabets@...arflare.com>,
        "maxime.coquelin@...hat.com" <maxime.coquelin@...hat.com>,
        "lingshan.zhu@...el.com" <lingshan.zhu@...el.com>,
        "dan.daly@...el.com" <dan.daly@...el.com>,
        "cunming.liang@...el.com" <cunming.liang@...el.com>,
        "zhihong.wang@...el.com" <zhihong.wang@...el.com>
Subject: Re: [PATCH] vhost: introduce vDPA based backend


On 2020/2/5 下午3:15, 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 上午11:36, Tiwei Bie wrote:
>>>> 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 named
>>>> 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@...el.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 during
>>> 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 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.


> Devices that has other ways to do the DMA mapping will call the proprietary APIs.


To confirm, do you prefer:

1) map/unmap

or

2) pass all maps at one time?

Thanks


>