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

[Vladimir Sementsov-Ogievskiy]

10.08.2020 15:35, 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.
>
> 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);

I think -v and -V should be mutually exclusive, as combined output doesn't make 
real sense.
Still, I'm OK with it as is (as well as with -V renamed to -r). I can suggest 
also -d (aka dump).

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

--
Best regards,
Vladimir