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

[Kevin Wolf <kwolf@redhat.com>]

Am 09.07.2012 11:36, schrieb Alexander Graf:
> 
> On 09.07.2012, at 11:27, Kevin Wolf wrote:
> 
>> Am 09.07.2012 11:13, schrieb Alexander Graf:
>>>
>>> On 09.07.2012, at 11:11, Kevin Wolf wrote:
>>>> Can we even make it the default with q35 as long as our AHCI controller
>>>> doesn't also expose a legacy interface for compatibility?
>>>
>>> What legacy interface? The ICH-9 can be controlled by the _BIOS_ to switch between 2 PCI IDs. One for IDE mode, one for AHCI mode. I don't think that would help us here, would it?
>>
>> I didn't actually look into this much. I just supposed that the
>> existence of an AHCI Enable bit in the GHC register implies that some
>> compatibility mechanism can be implemented that is transparent to older
>> OSes.
> 
> Yeah, I was hoping the same too when I read the spec back then. Unfortunately, it's really a firmware configuration bit which doesn't help us at all.
> 
> I think there is a hack in some driver somewhere that on bootup checks if the AHCI bit is disabled and then forcefully enables it, so that the device during OS boot magically changes its ID and semantics. But I don't think we really want to rely on that. IIRC it never went upstream and I doubt Windows does it.

Meh. So once again hardware doesn't work as you would expect.

>> If it can't, then I guess we can't change the meaning of -hda as long as
>> we claim that the command line is a stable API (or at least not if q35
>> is meant to become the default machine type at some point)
> 
> If -hda has the semantics of "create an IDE device", then no, we can't change it. It doesn't however. IIRC on -M pseries -hda creates SCSI devices. On s390 -hda creates virtio devices. So if on -M q35 -hda would create if=ahci, I don't see how that breaks the CLI.

It doesn't per se, that is as long as you need to explicitly specify -M
q35. But then changing the default machine from the existing pc to q35
would break the command line.

Kevin