Date:   Fri, 25 Dec 2020 15:02:11 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>, sgarzare@...hat.com,
        Parav Pandit <parav@...dia.com>, akpm@...ux-foundation.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        axboe@...nel.dk, bcrl@...ck.org, corbet@....net,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb
 message


On 2020/12/25 上午10:37, Yongji Xie wrote:
> On Thu, Dec 24, 2020 at 3:37 PM Yongji Xie <xieyongji@...edance.com> wrote:
>> On Thu, Dec 24, 2020 at 10:41 AM Jason Wang <jasowang@...hat.com> wrote:
>>>
>>> On 2020/12/23 下午8:14, Yongji Xie wrote:
>>>> On Wed, Dec 23, 2020 at 5:05 PM Jason Wang <jasowang@...hat.com> wrote:
>>>>> On 2020/12/22 下午10:52, Xie Yongji wrote:
>>>>>> To support vhost-vdpa bus driver, we need a way to share the
>>>>>> vhost-vdpa backend process's memory with the userspace VDUSE process.
>>>>>>
>>>>>> This patch tries to make use of the vhost iotlb message to achieve
>>>>>> that. We will get the shm file from the iotlb message and pass it
>>>>>> to the userspace VDUSE process.
>>>>>>
>>>>>> Signed-off-by: Xie Yongji <xieyongji@...edance.com>
>>>>>> ---
>>>>>>     Documentation/driver-api/vduse.rst |  15 +++-
>>>>>>     drivers/vdpa/vdpa_user/vduse_dev.c | 147 ++++++++++++++++++++++++++++++++++++-
>>>>>>     include/uapi/linux/vduse.h         |  11 +++
>>>>>>     3 files changed, 171 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/driver-api/vduse.rst b/Documentation/driver-api/vduse.rst
>>>>>> index 623f7b040ccf..48e4b1ba353f 100644
>>>>>> --- a/Documentation/driver-api/vduse.rst
>>>>>> +++ b/Documentation/driver-api/vduse.rst
>>>>>> @@ -46,13 +46,26 @@ The following types of messages are provided by the VDUSE framework now:
>>>>>>
>>>>>>     - VDUSE_GET_CONFIG: Read from device specific configuration space
>>>>>>
>>>>>> +- VDUSE_UPDATE_IOTLB: Update the memory mapping in device IOTLB
>>>>>> +
>>>>>> +- VDUSE_INVALIDATE_IOTLB: Invalidate the memory mapping in device IOTLB
>>>>>> +
>>>>>>     Please see include/linux/vdpa.h for details.
>>>>>>
>>>>>> -In the data path, VDUSE framework implements a MMU-based on-chip IOMMU
>>>>>> +The data path of userspace vDPA device is implemented in different ways
>>>>>> +depending on the vdpa bus to which it is attached.
>>>>>> +
>>>>>> +In virtio-vdpa case, VDUSE framework implements a MMU-based on-chip IOMMU
>>>>>>     driver which supports mapping the kernel dma buffer to a userspace iova
>>>>>>     region dynamically. The userspace iova region can be created by passing
>>>>>>     the userspace vDPA device fd to mmap(2).
>>>>>>
>>>>>> +In vhost-vdpa case, the dma buffer is reside in a userspace memory region
>>>>>> +which will be shared to the VDUSE userspace processs via the file
>>>>>> +descriptor in VDUSE_UPDATE_IOTLB message. And the corresponding address
>>>>>> +mapping (IOVA of dma buffer <-> VA of the memory region) is also included
>>>>>> +in this message.
>>>>>> +
>>>>>>     Besides, the eventfd mechanism is used to trigger interrupt callbacks and
>>>>>>     receive virtqueue kicks in userspace. The following ioctls on the userspace
>>>>>>     vDPA device fd are provided to support that:
>>>>>> diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
>>>>>> index b974333ed4e9..d24aaacb6008 100644
>>>>>> --- a/drivers/vdpa/vdpa_user/vduse_dev.c
>>>>>> +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
>>>>>> @@ -34,6 +34,7 @@
>>>>>>
>>>>>>     struct vduse_dev_msg {
>>>>>>         struct vduse_dev_request req;
>>>>>> +     struct file *iotlb_file;
>>>>>>         struct vduse_dev_response resp;
>>>>>>         struct list_head list;
>>>>>>         wait_queue_head_t waitq;
>>>>>> @@ -325,12 +326,80 @@ static int vduse_dev_set_vq_state(struct vduse_dev *dev,
>>>>>>         return ret;
>>>>>>     }
>>>>>>
>>>>>> +static int vduse_dev_update_iotlb(struct vduse_dev *dev, struct file *file,
>>>>>> +                             u64 offset, u64 iova, u64 size, u8 perm)
>>>>>> +{
>>>>>> +     struct vduse_dev_msg *msg;
>>>>>> +     int ret;
>>>>>> +
>>>>>> +     if (!size)
>>>>>> +             return -EINVAL;
>>>>>> +
>>>>>> +     msg = vduse_dev_new_msg(dev, VDUSE_UPDATE_IOTLB);
>>>>>> +     msg->req.size = sizeof(struct vduse_iotlb);
>>>>>> +     msg->req.iotlb.offset = offset;
>>>>>> +     msg->req.iotlb.iova = iova;
>>>>>> +     msg->req.iotlb.size = size;
>>>>>> +     msg->req.iotlb.perm = perm;
>>>>>> +     msg->req.iotlb.fd = -1;
>>>>>> +     msg->iotlb_file = get_file(file);
>>>>>> +
>>>>>> +     ret = vduse_dev_msg_sync(dev, msg);
>>>>> My feeling is that we should provide consistent API for the userspace
>>>>> device to use.
>>>>>
>>>>> E.g we'd better carry the IOTLB message for both virtio/vhost drivers.
>>>>>
>>>>> It looks to me for virtio drivers we can still use UPDAT_IOTLB message
>>>>> by using VDUSE file as msg->iotlb_file here.
>>>>>
>>>> It's OK for me. One problem is when to transfer the UPDATE_IOTLB
>>>> message in virtio cases.
>>>
>>> Instead of generating IOTLB messages for userspace.
>>>
>>> How about record the mappings (which is a common case for device have
>>> on-chip IOMMU e.g mlx5e and vdpa simlator), then we can introduce ioctl
>>> for userspace to query?
>>>
>> If so, the IOTLB UPDATE is actually triggered by ioctl, but
>> IOTLB_INVALIDATE is triggered by the message. Is it a little odd? Or
>> how about trigger it when userspace call mmap() on the device fd?
>>
> Oh sorry, looks like mmap() needs to be called in IOTLB UPDATE message
> handler. Is it possible for the vdpa device to know which vdpa bus it
> is attached to?


We'd better not. It's kind of layer violation.

Thanks


>   So that we can generate this message during probing.
> Otherwise, we don't know whether the iova domain of MMU-based IOMMU is
> needed.
>
> Thanks,
> Yongji
>

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