Date:   Fri, 25 Dec 2020 18:31:58 +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 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.

> 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.

Thanks,
Yongji

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