Re: [PATCH v6 2/5] softmmu/vl: Let -fw_cfg option take a 'blob_id' argument

[Laszlo Ersek <lersek@redhat.com>]

On 05/19/20 20:20, Philippe Mathieu-Daudé wrote:
> The 'blob_id' argument refers to a QOM object able to produce
> data consumable by the fw_cfg device. The producer object must
> implement the FW_CFG_DATA_GENERATOR interface.

OK, this answers my OBJECT_CHECK() question under patch #1 (in the
negative -- an assert would be wrong).

>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  softmmu/vl.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index ae5451bc23..f76c53ad2e 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -489,6 +489,10 @@ static QemuOptsList qemu_fw_cfg_opts = {
>              .name = "string",
>              .type = QEMU_OPT_STRING,
>              .help = "Sets content of the blob to be inserted from a string",
> +        }, {
> +            .name = "blob_id",
> +            .type = QEMU_OPT_STRING,
> +            .help = "Sets id of the object generating fw_cfg blob to be used",
>          },
>          { /* end of list */ }
>      },
> @@ -2020,7 +2024,7 @@ static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
>  {
>      gchar *buf;
>      size_t size;
> -    const char *name, *file, *str;
> +    const char *name, *file, *str, *blob_id;
>      FWCfgState *fw_cfg = (FWCfgState *) opaque;
>
>      if (fw_cfg == NULL) {
> @@ -2030,14 +2034,17 @@ static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
>      name = qemu_opt_get(opts, "name");
>      file = qemu_opt_get(opts, "file");
>      str = qemu_opt_get(opts, "string");
> +    blob_id = qemu_opt_get(opts, "blob_id");
>
>      /* we need name and either a file or the content string */

(1) Please update this comment. If the option is given, we need the
name, and exactly one of: file, content string, blob_id.

> -    if (!(nonempty_str(name) && (nonempty_str(file) || nonempty_str(str)))) {
> +    if (!(nonempty_str(name)
> +          && (nonempty_str(file) || nonempty_str(str) || nonempty_str(blob_id)))
> +         ) {
>          error_setg(errp, "invalid argument(s)");
>          return -1;
>      }

(2) Coding style: does QEMU keep operators on the left or on the right
when breaking subconditions to new lines? (I vaguely recall "to the
right", but I could be wrong... Well, "hw/nvram/fw_cfg.c" has at least 7
examples of the operator being on the right.)

> -    if (nonempty_str(file) && nonempty_str(str)) {
> -        error_setg(errp, "file and string are mutually exclusive");
> +    if (nonempty_str(file) && nonempty_str(str) && nonempty_str(blob_id)) {
> +        error_setg(errp, "file, string and blob_id are mutually exclusive");
>          return -1;
>      }

(3) I believe this catches only when all three of name/string/blob_id
are given. But we should continue catching "two given".

How about reworking both "if"s, *and* the comment at (1) at the same
time, into:

    if (!nonempty_str(name) ||
        nonempty_str(file) + nonempty_str(str) + nonempty_str(blob_id) != 1) {
        error_setg(errp, "name, plus exactly one of file, string and blob_id, "
                   "are needed");
        return -1;
    }

(Regarding the addition, nonempty_str() returns a "bool", which is a
macro to _Bool, which is promoted to "int" or "unsigned int".)

>      if (strlen(name) > FW_CFG_MAX_FILE_PATH - 1) {
> @@ -2052,6 +2059,8 @@ static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
>      if (nonempty_str(str)) {
>          size = strlen(str); /* NUL terminator NOT included in fw_cfg blob */
>          buf = g_memdup(str, size);
> +    } else if (nonempty_str(blob_id)) {
> +        return fw_cfg_add_from_generator(fw_cfg, name, blob_id, errp);
>      } else {
>          GError *err = NULL;
>          if (!g_file_get_contents(file, &buf, &size, &err)) {
>

(4) The "-fw_cfg" command line option is documented in both the qemu(1)
manual, and the "docs/specs/fw_cfg.txt" file.

I think we may have to update those. In particular I mean *where* the
option is documented (in both texts).

In the manual, "-fw_cfg" is currently under "Debug/Expert options", but
that will no longer apply (I think?) after this series.

Similarly, in "docs/specs/fw_cfg.txt", the section is called "Externally
Provided Items" -- but that might not be strictly true any more either.

Maybe leave the current "-fw_cfg" mentions in peace, and document
"-fw_cfg blob_id=..." separately (in different docs sections)? The
"fw_cfg generators" concept could deserve dedicated sections.

Sorry that I can't make a good concrete suggestion. :(

Thanks,
Laszlo