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

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Message-ID: <80b4a5f9-8cc0-326a-a133-07a0ae3c7909@redhat.com>
Date:   Thu, 6 Feb 2020 11:09:42 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Shahaf Shuler <shahafs@...lanox.com>
Cc:     Tiwei Bie <tiwei.bie@...el.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6:33, Michael S. Tsirkin wrote:
> On Wed, Feb 05, 2020 at 09:30:14AM +0000, Shahaf Shuler wrote:
>> Wednesday, February 5, 2020 9:50 AM, Jason Wang:
>>> 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.
>> Need to see the layering of such proposal but am not sure.
>> Vhost-vdpa is generic framework, while the DMA mapping is vendor specific.
>> Maybe vhost-vdpa can have some shared code needed to operate on iommu, so drivers can re-use it.  to me it seems simpler than exposing a new iommu device.
>>
>>>> Devices that has other ways to do the DMA mapping will call the
>>> proprietary APIs.
>>>
>>>
>>> To confirm, do you prefer:
>>>
>>> 1) map/unmap
>> It is not only that. AFAIR there also flush and invalidate calls, right?
>>
>>> or
>>>
>>> 2) pass all maps at one time?
>> To me this seems more straight forward.
>> It is correct that under hotplug and large number of memory segments
>> the driver will need to understand the diff (or not and just reload
>> the new configuration).
>> However, my assumption here is that memory
>> hotplug is heavy flow anyway, and the driver extra cycles will not be
>> that visible
> I think we can just allow both, after all vhost already has both interfaces ...
> We just need a flag that tells userspace whether it needs to
> update all maps aggressively or can wait for a fault.


It looks to me such flag is not a must and we can introduce it later 
when device support page fault.

Thanks