Re: [PATCH 04/11] qemu-options: finesse the recommendations around -blockdev

[Thomas Huth]

On 03/04/2023 16.55, Markus Armbruster wrote:
> Alex Bennée <alex.bennee@linaro.org> writes:
>
> > Markus Armbruster <armbru@redhat.com> writes:
> >
> > > Alex Bennée <alex.bennee@linaro.org> writes:
...
> > I was under the impression things like -hda wouldn't work say on an Arm
> > machine because you don't know what sort of interface you might be
> > using and -hda implies IDE. Where is this macro substitution done?
>
> qemu_init() calls drive_add() for all these options.
>
> drive_add(TYPE, INDEX, FILE, OPTSTR) creates a QemuOpts in group
> "drive".  It sets "if" to if_name[TYPE] unless TYPE is IF_DEFAULT,
> "index" to INDEX unless it's negative, and "file" to FILE unless it's
> null.  Then it parses OPTSTR on top.
>
> For -hdX, the call looks like
>
>                  drive_add(IF_DEFAULT, popt->index - QEMU_OPTION_hda, optarg,
>                            HD_OPTS);
>
> We pass IF_DEFAULT, so "if" remains unset.  "index" is set to 0 for
> -hda, 1, for -hdb and so forth.  "file" is set to the option argument.
> Since HD_OPTS is "media=disk", we set "media" to "disk".
>
> The QemuOpts in config group "drive" get passed to drive_new() via
> drive_init_func().  Unset "if" defaults to the current machine's class's
> block_default_type.
>
> If a machine doesn't set this member explicitly, it remains zero, which
> is IF_NONE.  Documented in blockdev.h:
>
>      typedef enum {
>          IF_DEFAULT = -1,            /* for use with drive_add() only */
>          /*
>           * IF_NONE must be zero, because we want MachineClass member
> --->     * block_default_type to default-initialize to IF_NONE
>           */
>          IF_NONE = 0,
>          IF_IDE, IF_SCSI, IF_FLOPPY, IF_PFLASH, IF_MTD, IF_SD, IF_VIRTIO, 
> IF_XEN,
>          IF_COUNT
>      } BlockInterfaceType;
>
> Questions?

How's the average user supposed to know that? Our qemu-options.hx just says: 
"-hda/-hdb file  use 'file' as IDE hard disk 0/1 image"...

 Thomas