Re: [Qemu-devel] [PATCHv2] block: add native support for NFS

[Stefan Hajnoczi <stefanha@redhat.com>]

On Fri, Dec 20, 2013 at 10:48:41AM +0100, Peter Lieven wrote:
> On 17.12.2013 17:47, Stefan Hajnoczi wrote:
> >On Tue, Dec 17, 2013 at 10:15:25AM +0100, Peter Lieven wrote:
> >>This patch adds native support for accessing images on NFS shares without
> >>the requirement to actually mount the entire NFS share on the host.
> >>
> >>NFS Images can simply be specified by an url of the form:
> >>nfs://<host>/<export>/<filename>
> >>
> >>For example:
> >>qemu-img create -f qcow2 nfs://10.0.0.1/qemu-images/test.qcow2
> >>
> >>You need libnfs from Ronnie Sahlberg available at:
> >>    git://github.com/sahlberg/libnfs.git
> >>for this to work.
> >>
> >>During configure it is automatically probed for libnfs and support
> >>is enabled on-the-fly. You can forbid or enforce libnfs support
> >>with --disable-libnfs or --enable-libnfs respectively.
> >>
> >>Due to NFS restrictions you might need to execute your binaries
> >>as root, allow them to open priviledged ports (<1024) or specify
> >>insecure option on the NFS server.
> >>
> >>Signed-off-by: Peter Lieven <pl@kamp.de>
> >>---
> >>v1->v2:
> >>  - fixed block/Makefile.objs [Ronnie]
> >>  - do not always register a read handler [Ronnie]
> >>  - add support for reading beyond EOF [Fam]
> >>  - fixed struct and paramter naming [Fam]
> >>  - fixed overlong lines and whitespace errors [Fam]
> >>  - return return status from libnfs whereever possible [Fam]
> >>  - added comment why we set allocated_file_size to -ENOTSUP after write [Fam]
> >>  - avoid segfault when parsing filname [Fam]
> >>  - remove unused close_bh from NFSClient [Fam]
> >>  - avoid dividing and mutliplying total_size by BDRV_SECTOR_SIZE in nfs_file_create [Fam]
> >>
> >>  MAINTAINERS         |    5 +
> >>  block/Makefile.objs |    1 +
> >>  block/nfs.c         |  419 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >>  configure           |   38 +++++
> >>  4 files changed, 463 insertions(+)
> >>  create mode 100644 block/nfs.c
> >Which NFS protocol versions are supported by current libnfs?
> >
> >>+#include <poll.h>
> >Why is this header included?
> >
> >>+typedef struct nfsclient {
> >Please either drop the struct tag or use "NFSClient".
> >
> >>+static void
> >>+nfs_co_generic_cb(int status, struct nfs_context *nfs, void *data,
> >>+                  void *private_data)
> >>+{
> >>+    NFSTask *Task = private_data;
> >lowercase "task" local variable name please.
> >
> >>+static int coroutine_fn nfs_co_writev(BlockDriverState *bs,
> >>+                                        int64_t sector_num, int nb_sectors,
> >>+                                        QEMUIOVector *iov)
> >>+{
> >>+    NFSClient *client = bs->opaque;
> >>+    NFSTask task;
> >>+    char *buf = NULL;
> >>+
> >>+    nfs_co_init_task(client, &task);
> >>+
> >>+    buf = g_malloc(nb_sectors * BDRV_SECTOR_SIZE);
> >>+    qemu_iovec_to_buf(iov, 0, buf, nb_sectors * BDRV_SECTOR_SIZE);
> >>+
> >>+    if (nfs_pwrite_async(client->context, client->fh,
> >>+                         sector_num * BDRV_SECTOR_SIZE,
> >>+                         nb_sectors * BDRV_SECTOR_SIZE,
> >>+                         buf, nfs_co_generic_cb, &task) != 0) {
> >>+        g_free(buf);
> >>+        return -EIO;
> >Can we get a more detailed errno here?  (e.g. ENOSPC)
> >
> >>+    }
> >>+
> >>+    while (!task.complete) {
> >>+        nfs_set_events(client);
> >>+        qemu_coroutine_yield();
> >>+    }
> >>+
> >>+    g_free(buf);
> >>+
> >>+    if (task.status != nb_sectors * BDRV_SECTOR_SIZE) {
> >>+        return task.status < 0 ? task.status : -EIO;
> >>+    }
> >>+
> >>+    bs->total_sectors = MAX(bs->total_sectors, sector_num + nb_sectors);
> >Why is this necessary?  block.c will update bs->total_sectors if the
> >file is growable.
> >
> >>+    /* set to -ENOTSUP since bdrv_allocated_file_size is only used
> >>+     * in qemu-img open. So we can use the cached value for allocate
> >>+     * filesize obtained from fstat at open time */
> >>+    client->allocated_file_size = -ENOTSUP;
> >Can you implement this fully?  By stubbing it out like this we won't be
> >able to call get_allocated_file_size() at runtime in the future without
> >updating the nfs block driver code.  It's just an fstat call, shouldn't
> >be too hard to implement properly :).
> 
> It seems I have to leave it as is currently. bdrv_get_allocated_file_size
> is not in a coroutine context. I get coroutine yields to no one.

Create a coroutine and pump the event loop until it has reached
completion:

co = qemu_coroutine_create(my_coroutine_fn, ...);
qemu_coroutine_enter(co, foo);
while (!complete) {
    qemu_aio_wait();
}

See block.c for similar examples.

Stefan