Re: [PATCH v2 0/5] VDUSE: Support registering userspace memory as bounce buffer

Yongji Xie
2022-07-06T10:16:13Z

On Wed, Jul 6, 2022 at 5:30 PM Jason Wang <jasowang@redhat.com> wrote:
>
> On Wed, Jul 6, 2022 at 1:05 PM Xie Yongji <xieyongji@bytedance.com> wrote:
> >
> > Hi all,
> >
> > This series introduces some new ioctls: VDUSE_IOTLB_GET_INFO,
> > VDUSE_IOTLB_REG_UMEM and VDUSE_IOTLB_DEREG_UMEM to support
> > registering and de-registering userspace memory for IOTLB
> > as bounce buffer in virtio-vdpa case.
> >
> > The VDUSE_IOTLB_GET_INFO ioctl can help user to query IOLTB
> > information such as bounce buffer size. Then user can use
> > those information on VDUSE_IOTLB_REG_UMEM and
> > VDUSE_IOTLB_DEREG_UMEM ioctls to register and de-register
> > userspace memory for IOTLB.
> >
> > During registering and de-registering, the DMA data in use
> > would be copied from kernel bounce pages to userspace bounce
> > pages and back.
> >
> > With this feature, some existing application such as SPDK
> > and DPDK can leverage the datapath of VDUSE directly and
> > efficiently as discussed before [1][2]. They can register
> > some preallocated hugepages to VDUSE to avoid an extra
> > memcpy from bounce-buffer to hugepages.
>
> This is really interesting.
>
> But a small concern on uAPI is that this seems to expose the VDUSE
> internal implementation (bounce buffer) to userspace. We tried hard to
> hide it via the GET_FD before. Anyway can we keep it?
>

Another way is changing GET_FD ioctl to add a flag or reuse 'perm'
field to indicate whether a IOVA region supports userspace memory
registration. Then userspace can use
VDUSE_IOTLB_REG_UMEM/VDUSE_IOTLB_DEREG_UMEM to register/deregister
userspace memory for this IOVA region. Any suggestions?

Thanks,
Yongji