Virtualization - Dec 2020 - [RFC v2 09/13] vduse: Add support for processing vhost iotlb message

On 2020/12/24 ??3:37, Yongji Xie wrote:
> On Thu, Dec 24, 2020 at 10:41 AM Jason Wang <jasowang at redhat.com>
wrote:
>>
>> On 2020/12/23 ??8:14, Yongji Xie wrote:
>>> On Wed, Dec 23, 2020 at 5:05 PM Jason Wang <jasowang at
redhat.com> wrote:
>>>> On 2020/12/22 ??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 at
bytedance.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?

Good point.

Some questions here:

1) Userspace think the IOTLB was flushed after IOTLB_INVALIDATE syscall 
is returned. But this patch doesn't have this guarantee. Can this lead 
some issues?
2) I think after VDUSE userspace receives IOTLB_INVALIDATE, it needs to 
issue a munmap(). What if it doesn't do that?

>   Or
> how about trigger it when userspace call mmap() on the device fd?

One possible issue is that the IOTLB_UPDATE/INVALIDATE might come after 
mmap():

1) When vIOMMU is enabled
2) Guest memory topology has been changed (memory hotplug).

Thanks

>
> Thanks,
> Yongji
>