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

[Stefano Garzarella]

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?

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);
+}
+
 static int qemu_lock_fcntl(int fd, int64_t start, int64_t len, int fl_type)
 {
     int ret;

Thanks,
Stefano