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

[Markus Armbruster]

Thomas Huth <thuth@redhat.com> writes:

> 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"...

Ancient doc bug.  Should have been updated in commit 2d0d2837dcf
(Support default block interfaces per QEMUMachine) back in 2012.