Re: [PATCH for 5.2 1/1] qemu-io: add -V flag for read sub-command

["Richard W.M. Jones" <rjones@redhat.com>]

On Mon, Aug 10, 2020 at 03:35:55PM +0300, Denis V. Lunev wrote:
> The problem this patch is trying to address is libguestfs behavior on the
> appliance startup. It starts supporting to use root=UUID definition in
> the kernel command line of its root filesystem using
>     file --  /usr/lib64/guestfs/appliance/root
> This works fine with RAW image, but we are using QCOW2 as a storage to
> save a bit of file space and in this case we get
>     QEMU QCOW Image (v3), 1610612736 bytes
> instead of UUID of the root filesystem.
> 
> The solution is very simple - we should dump first 256k of the image file
> like the follows
>     qemu-io -c "read -V 0 256k" appliance | file -
> which will provide correct result for all possible types of the appliance
> storage.
> 
> Unfortunately, additional option for qemu-io is the only and the simplest
> solution as '-v' creates very specific output, which requires to be
> parsed. 'qemu-img dd of=/dev/stdout' does not work and the fix would be
> much more intrusive.

I like the idea of the flag - we could also use it in the nbdkit test
suite.

I wonder if the actual flag ('V') is a good idea because I would
normally associate that with getting the version of a command.  But
as these are subcommand flags, that's probably not too relevant here.

So I think this is fine from my point of view.

Rich.

> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
> CC: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Richard W.M. Jones <rjones@redhat.com>
> ---
> P.S. Patch to libguestfs will follow.
> 
>  qemu-io-cmds.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/qemu-io-cmds.c b/qemu-io-cmds.c
> index baeae86d8c..7aae9726cd 100644
> --- a/qemu-io-cmds.c
> +++ b/qemu-io-cmds.c
> @@ -718,7 +718,7 @@ static const cmdinfo_t read_cmd = {
>      .cfunc      = read_f,
>      .argmin     = 2,
>      .argmax     = -1,
> -    .args       = "[-abCqv] [-P pattern [-s off] [-l len]] off len",
> +    .args       = "[-abCqvV] [-P pattern [-s off] [-l len]] off len",
>      .oneline    = "reads a number of bytes at a specified offset",
>      .help       = read_help,
>  };
> @@ -728,6 +728,7 @@ static int read_f(BlockBackend *blk, int argc, char **argv)
>      struct timespec t1, t2;
>      bool Cflag = false, qflag = false, vflag = false;
>      bool Pflag = false, sflag = false, lflag = false, bflag = false;
> +    bool vrawflag = true;
>      int c, cnt, ret;
>      char *buf;
>      int64_t offset;
> @@ -737,7 +738,7 @@ static int read_f(BlockBackend *blk, int argc, char **argv)
>      int pattern = 0;
>      int64_t pattern_offset = 0, pattern_count = 0;
>  
> -    while ((c = getopt(argc, argv, "bCl:pP:qs:v")) != -1) {
> +    while ((c = getopt(argc, argv, "bCl:pP:qs:vV")) != -1) {
>          switch (c) {
>          case 'b':
>              bflag = true;
> @@ -777,6 +778,9 @@ static int read_f(BlockBackend *blk, int argc, char **argv)
>          case 'v':
>              vflag = true;
>              break;
> +        case 'V':
> +            vrawflag = true;
> +            break;
>          default:
>              qemuio_command_usage(&read_cmd);
>              return -EINVAL;
> @@ -869,10 +873,15 @@ static int read_f(BlockBackend *blk, int argc, char **argv)
>      if (vflag) {
>          dump_buffer(buf, offset, count);
>      }
> +    if (vrawflag) {
> +        write(STDOUT_FILENO, buf, count);
> +    }
>  
>      /* Finally, report back -- -C gives a parsable format */
> -    t2 = tsub(t2, t1);
> -    print_report("read", &t2, offset, count, total, cnt, Cflag);
> +    if (!vrawflag) {
> +        t2 = tsub(t2, t1);
> +        print_report("read", &t2, offset, count, total, cnt, Cflag);
> +    }
>  
>  out:
>      qemu_io_free(buf);
> -- 
> 2.17.1

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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