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

[Stefan Hajnoczi]

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 <address@hidden>
> ---
> 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 :).

> +    if (client->context == NULL) {
> +        error_setg(errp, "Failed to init NFS context");
> +        ret = -EINVAL;
> +        goto fail;
> +    }
> +
> +    if (strlen(filename) <= 6) {
> +        error_setg(errp, "Invalid server in URL");
> +        ret = -EINVAL;
> +        goto fail;
> +    }
> +
> +    server = g_strdup(filename + 6);
> +    if (server[0] == '/' || server[0] == '\0') {
> +        error_setg(errp, "Invalid server in URL");
> +        ret = -EINVAL;
> +        goto fail;
> +    }
> +    strp = strchr(server, '/');
> +    if (strp == NULL) {
> +        error_setg(errp, "Invalid URL specified.\n");
> +        ret = -EINVAL;
> +        goto fail;
> +    }
> +    path = g_strdup(strp);
> +    *strp = 0;
> +    strp = strrchr(path, '/');
> +    if (strp == NULL) {
> +        error_setg(errp, "Invalid URL specified.\n");
> +        ret = -EINVAL;
> +        goto fail;
> +    }
> +    file = g_strdup(strp);
> +    *strp = 0;

Can you use util/uri.c to avoid the string manipulation?  It can extract
the different parts for validation: scheme, server, port, and path.

> +static int nfs_file_create(const char *filename, QEMUOptionParameter 
> *options,
> +                         Error **errp)
> +{
> +    int ret = 0;
> +    int64_t total_size = 0;
> +    BlockDriverState *bs;
> +    NFSClient *client = NULL;
> +    QDict *bs_options;
> +
> +    bs = bdrv_new("");

This approach seems a little risky to me.  bs is a fake
BlockDriverState with many fields not initialized.  bdrv_*() calls would
crash.

How about a static nfs_open() function that operates on NFSClient and is
shared by nfs_file_create() and nfs_file_open()?

> +##########################################
> +# Do we have libnfs
> +if test "$libnfs" != "no" ; then
> +  cat > $TMPC << EOF
> +#include <nfsc/libnfs-zdr.h>
> +#include <nfsc/libnfs.h>
> +#include <nfsc/libnfs-raw.h>
> +#include <nfsc/libnfs-raw-mount.h>
> +int main(void) {
> +    nfs_init_context();
> +    nfs_pread_async(0,0,0,0,0,0);
> +    nfs_pwrite_async(0,0,0,0,0,0,0);
> +    nfs_fstat(0,0,0);
> +    return 0;
> +    }
> +EOF
> +  if compile_prog "-Werror" "-lnfs" ; then
> +    libnfs="yes"
> +    LIBS="$LIBS -lnfs"

pkg-config is usually better than hardcoding names.  Is pkg-config
available for libnfs?