Date:   Mon, 28 Dec 2020 16:14:14 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.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: Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb message

On Mon, Dec 28, 2020 at 3:43 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> On 2020/12/25 下午6:31, Yongji Xie wrote:
> > On Fri, Dec 25, 2020 at 2:58 PM Jason Wang <jasowang@...hat.com> wrote:
> >>
> >> On 2020/12/24 下午3:37, Yongji Xie 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?
> >>
> >> 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?
> > I'm not sure. But should it be guaranteed in VDUSE userspace? Just
> > like what vhost-user backend process does.
>
>
> I think so. This is because the userspace device needs a way to
> synchronize invalidation with its datapath. Otherwise, guest may thing
> the buffer is freed to be used by other parts but the it actually can be
> used by the VDUSE device. The may cause security issues.
>
>
> >
> >> 2) I think after VDUSE userspace receives IOTLB_INVALIDATE, it needs to
> >> issue a munmap(). What if it doesn't do that?
> >>
> > Yes, the munmap() is needed. If it doesn't do that, VDUSE userspace
> > could still access the corresponding guest memory region.
>
>
> I see. So all the above two questions are because VHOST_IOTLB_INVALIDATE
> is expected to be synchronous. This need to be solved by tweaking the
> current VDUSE API or we can re-visit to go with descriptors relaying first.
>

Actually all vdpa related operations are synchronous in current
implementation. The ops.set_map/dma_map/dma_unmap should not return
until the VDUSE_UPDATE_IOTLB/VDUSE_INVALIDATE_IOTLB message is replied
by userspace. Could it solve this problem?

Thanks,
Yongji

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