Re: [PATCH] block/blkio: add 'fd' option to virtio-blk-vhost-vdpa driver

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, May 03, 2023 at 11:15:56AM +0200, Stefano Garzarella wrote:
> On Tue, May 02, 2023 at 03:02:32PM -0400, Stefan Hajnoczi wrote:
> > On Tue, May 02, 2023 at 04:50:50PM +0200, Stefano Garzarella wrote:
> > > The virtio-blk-vhost-vdpa driver in libblkio 1.3.0 supports the new
> > > 'fd' property. Let's expose this to the user, so the management layer
> > > can pass the file descriptor of an already opened vhost-vdpa character
> > > device. This is useful especially when the device can only be accessed
> > > with certain privileges.
> > > 
> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > ---
> > > 
> > > Notes:
> > >     As an alternative we could support passing `/dev/fdset/N` via 'path',
> > >     always opening the path with qemu_open() and passing the fd to the
> > >     libblkio driver.
> > >     I preferred to add a new parameter though, because the code is
> > >     simpler without changing how path works (alternatively we should check
> > >     first if fd is supported by the driver or not).
> > > 
> > >     What do you think?
> > 
> > I think the approach in this patch is fine.
> > 
> > > 
> > >     Thanks,
> > >     Stefano
> > > 
> > >  qapi/block-core.json |  6 +++++-
> > >  block/blkio.c        | 45 +++++++++++++++++++++++++++++++++++++++++++-
> > >  2 files changed, 49 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/qapi/block-core.json b/qapi/block-core.json
> > > index b57978957f..9f70777d49 100644
> > > --- a/qapi/block-core.json
> > > +++ b/qapi/block-core.json
> > > @@ -3841,10 +3841,14 @@
> > >  #
> > >  # @path: path to the vhost-vdpa character device.
> > >  #
> > > +# @fd: file descriptor of an already opened vhost-vdpa character device.
> > > +#      (Since 8.1)
> > > +#
> > >  # Since: 7.2
> > >  ##
> > >  { 'struct': 'BlockdevOptionsVirtioBlkVhostVdpa',
> > > -  'data': { 'path': 'str' },
> > > +  'data': { '*path': 'str',
> > > +            '*fd': 'str' },
> > >    'if': 'CONFIG_BLKIO' }
> > > 
> > >  ##
> > > diff --git a/block/blkio.c b/block/blkio.c
> > > index 0cdc99a729..98394b5745 100644
> > > --- a/block/blkio.c
> > > +++ b/block/blkio.c
> > > @@ -694,6 +694,49 @@ static int blkio_virtio_blk_common_open(BlockDriverState *bs,
> > >      return 0;
> > >  }
> > > 
> > > +static int blkio_virtio_blk_vhost_vdpa_open(BlockDriverState *bs,
> > > +        QDict *options, int flags, Error **errp)
> > > +{
> > > +    const char *path = qdict_get_try_str(options, "path");
> > > +    const char *fd_str = qdict_get_try_str(options, "fd");
> > > +    BDRVBlkioState *s = bs->opaque;
> > > +    int ret;
> > > +
> > > +    if (path && fd_str) {
> > > +        error_setg(errp, "'path' and 'fd' options are mutually exclusive");
> > > +        return -EINVAL;
> > > +    }
> > > +
> > > +    if (!path && !fd_str) {
> > > +        error_setg(errp, "none of 'path' or 'fd' options was specified");
> > > +        return -EINVAL;
> > > +    }
> > > +
> > > +    if (path) {
> > > +        ret = blkio_set_str(s->blkio, "path", path);
> > > +        qdict_del(options, "path");
> > > +        if (ret < 0) {
> > > +            error_setg_errno(errp, -ret, "failed to set path: %s",
> > > +                             blkio_get_error_msg());
> > > +            return ret;
> > > +        }
> > > +    } else {
> > > +        ret = blkio_set_str(s->blkio, "fd", fd_str);
> > 
> > monitor_fd_param() is used by vhost-net, vhost-vsock, vhost-scsi, etc.
> > 
> > I think QEMU should parse the fd string and resolve it to a file
> > descriptor so the fd passing syntax matches the other vhost devices.
> 
> Okay, but I have a linker issue if I use monitor_fd_param().
> IIUC because blkio is built as a module, so what about adding
> qemu_fd_param() in libqemuutil?

Modules can access any extern function in QEMU so I don't think there is
a fundamental limitation there.

Maybe it's related to the dependencies between the blkio module and
monitor/ code. monitor_get_fd_param() is in softmmu_ss, which block
drivers don't directly depend on AFAICT.

> 
> I mean something like this:
> 
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 9eff0be95b..87360c983a 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -568,6 +568,7 @@ int qemu_lock_fd(int fd, int64_t start, int64_t len, bool exclusive);
>  int qemu_unlock_fd(int fd, int64_t start, int64_t len);
>  int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive);
>  bool qemu_has_ofd_lock(void);
> +int qemu_fd_param(const char *fdname, Error **errp);
>  #endif
> 
>  #if defined(__HAIKU__) && defined(__i386__)
> diff --git a/util/osdep.c b/util/osdep.c
> index e996c4744a..ed0832810b 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -234,6 +234,11 @@ bool qemu_has_ofd_lock(void)
>  #endif
>  }
> 
> +int qemu_fd_param(const char *fdname, Error **errp)
> +{
> +    return monitor_fd_param(monitor_cur(), fdname, errp);
> +}

I'm not sure. If it works with modules enabled/disabled,
qemu-io/qemu-img/etc, and qemu-user then I guess this solution is okay.

Sorry I don't know the answer!

Stefan

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