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

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Date:   Fri, 8 Jul 2022 16:38:37 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     mst <mst@...hat.com>, Liu Xiaodong <xiaodong.liu@...el.com>,
        Maxime Coquelin <maxime.coquelin@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        songmuchun@...edance.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/5] VDUSE: Support registering userspace memory as
 bounce buffer

On Wed, Jul 6, 2022 at 6:16 PM Yongji Xie <xieyongji@...edance.com> wrote:
>
> On Wed, Jul 6, 2022 at 5:30 PM Jason Wang <jasowang@...hat.com> wrote:
> >
> > On Wed, Jul 6, 2022 at 1:05 PM Xie Yongji <xieyongji@...edance.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.

Looks better.

> Any suggestions?

I wonder what's the value of keeping the compatibility with the kernel
mmaped bounce buffer. It means we need to take extra care on e.g data
copying when reg/reg user space memory.

Can we simply allow the third kind of fd that only works for umem registration?

Thanks

>
> Thanks,
> Yongji
>