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

[Kevin Wolf]

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