Re: [Qemu-devel] [PATCH v2] ahci: add -drive support

[Markus Armbruster]

Alexander Graf <address@hidden> writes:

> On 12.07.2012, at 10:17, Markus Armbruster wrote:
>
>> [Alex's illegibly long lines wrapped]
>> 
>> Alexander Graf <address@hidden> writes:
>> 
>>> On 09.07.2012, at 10:50, Markus Armbruster wrote:
>>> 
>>>> Alexander Graf <address@hidden> writes:
>>>> 
>>>>> We've had support for creating AHCI devices using -device for a while now,
>>>>> but it's cumbersome to users. We really should provide an easier way for
>>>>> them to leverage the power of AHCI!
>>>>> 
>>>>> So let's introduce a new if= option to -drive, giving users the same
>>>>> command line experience as for scsi or ide.
>>>>> 
>>>>> Signed-off-by: Alexander Graf <address@hidden>
>>>>> 
>>>>> ---
>>>>> 
>>>>> v1 -> v2:
>>>>> 
>>>>> - support more than a single drive per adapter
>>>>> - support index= option
>>>>> - treat IF_AHCI the same as IF_IDE
>>>> 
>>>> Inhowfar?  Not obvious to me from the patch, or the diff patch v1.
>>>> 
>>>>> - add is_ata() helper to match AHCI || IDE
>>>> 
>>>> Not addressed:
>>>> 
>>>> Once we switch to q35, if=ahci will become a redundant wart: to add
>>>> drives to the board's AHCI controller, you'll have to use if=ide.
>>>> if=ahci will create new controllers, which is generally not what you
>>>> want.  Ugh.
>>>> 
>>>> 
>>>>> ---
>>>>> blockdev.c      |   54 
>>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++----
>>>>> blockdev.h      |    7 +++++++
>>>>> qemu-options.hx |    7 ++++++-
>>>>> vl.c            |    2 ++
>>>>> 4 files changed, 65 insertions(+), 5 deletions(-)
>>>>> 
>>>>> diff --git a/blockdev.c b/blockdev.c
>>>>> index 9e0a72a..744a886 100644
>>>>> --- a/blockdev.c
>>>>> +++ b/blockdev.c
>>>>> @@ -32,6 +32,7 @@ static const char *const if_name[IF_COUNT] = {
>>>>>    [IF_SD] = "sd",
>>>>>    [IF_VIRTIO] = "virtio",
>>>>>    [IF_XEN] = "xen",
>>>>> +    [IF_AHCI] = "ahci",
>>>>> };
>>>>> 
>>>>> static const int if_max_devs[IF_COUNT] = {
>>>>> @@ -51,6 +52,7 @@ static const int if_max_devs[IF_COUNT] = {
>>>>>     */
>>>>>    [IF_IDE] = 2,
>>>>>    [IF_SCSI] = 7,
>>>>> +    [IF_AHCI] = 6,
>>>>> };
>>>>> 
>>>>> /*
>>>>> @@ -330,15 +332,15 @@ DriveInfo *drive_init(QemuOpts *opts, int 
>>>>> default_to_scsi)
>>>>      if ((buf = qemu_opt_get(opts, "if")) != NULL) {
>>>>          for (type = 0; type < IF_COUNT && strcmp(buf, if_name[type]); 
>>>> type++)
>>>>              ;
>>>>          if (type == IF_COUNT) {
>>>>              error_report("unsupported bus type '%s'", buf);
>>>>              return NULL;
>>>>          }
>>>>      } else {
>>>>          type = default_to_scsi ? IF_SCSI : IF_IDE;
>>>>      }
>>>> 
>>>> A board can't default to IF_AHCI.  I guess what such a board would do is
>>>> treat IF_IDE and IF_AHCI just the same.
>>>> 
>>>> Leads me this question: what do "if=ide", "if=ahci" and "no if given"
>>>> mean?  Let me try:
>>>> 
>>>> * "if=ide" means "if the board provides an IDE controller, create an IDE
>>>> device attached to it.  What kind of IDE controller the board provides
>>>> doesn't matter.  In particular, an AHCI controller is fine.
>>> 
>>> I don't think this is what we want it to mean. What we want is:
>>> 
>>> "if=ide" means "if the board provides an IDE controller, create an IDE
>>> device attached to it. If it does not provide one, create one".
>> 
>> Okay, we got two competing ideas here.
>> 
>> 1. -drive doesn't give you any control over the kind of IDE controller.
>> You can select an IDE bus by number (bus=...), and you get whatever
>> existing controller provides this bus.  If none exists, a board-specific
>> one may be created for your convenience.  If you need more control, use
>> -device to set up controllers the way you want.
>> 
>> 2. -drive gives you control over AHCI (if=ahci) vs. "other" (if=ide).
>> IDE buses are numbered separately for if=ahci and if=ide.  With if=ahci,
>> you get an existing AHCI controller.  If none exists, a board-specific
>> one may be created for your convenience.  With if=ide, you get an
>> existing non-AHCI controller.  If none exists, a board-specific one may
>> be created for your convenience.  If you need more control, use -device
>> to set up controllers the way you want.
>> 
>> The question to answer is whether the difference between AHCI and
>> non-AHCI is so important that we want to provide control in -drive.
>> 
>> What I'd like to avoid is casual users setting up new guests with our
>> shiny new q35 board getting their IDE drives connected to some slow, old
>> controller just because they haven't discovered that if=ide doesn't cut
>> it anymore, and they need to use if=ahci now.
>
> Ah, I think I understand the main issue now: You're thinking of AHCI
> as an IDE controller.
>
> It isn't. AHCI is on the same level as IDE. They both speak ATA, but
> the guest os interface is completely different. You can write a
> generic IDE driver, but that won't be able to talk to an AHCI
> controller in AHCI mode. You can write a generic AHCI driver, but that
> won't be able to talk to an IDE controller.

Yes, but does the naive command line user care?

-serial configures a serial device.  The kind of device depends on the
board: 16550A UARTs with -M pc, Exynos 4210 UARTs with -M nuri, ColdFire
UARTs with -M an5206, ...  You can't write a generic serial device
driver.

Any thoughts on the remainder of my message, behavior of if=ahci in
particular?