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

[Kevin Wolf <kwolf@redhat.com>]

Am 10.08.2020 um 16:02 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 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).

I like -d (maybe with an alias --dump), though in the end the naming is
secondary. I think having some flag like this is very useful.

How about adding the same thing to write, i.e. get the buffer to write
from stdin?

Kevin