Re: [Qemu-devel] Question about xen disk unplug support for ahci missed in qemu

[John Snow]

On 10/19/2015 07:44 AM, Stefano Stabellini wrote:
> On Mon, 19 Oct 2015, Gerd Hoffmann wrote:
>>   Hi,
>>
>>>> I'm trying to follow this discussion as best as I am able, but my lack
>>>> of experience with Xen prevents me from really participating in a
>>>> meaningful way.
>>>>
>>>> (I see that Laszlo is still discussing some CD-ROM issues with Fabio
>>>> which may be of interest to me...)
>>>>
>>>> At any rate, I won't be authoring any Xen-specific hacks to the AHCI
>>>> device, but I do have plans to implement hot-plugging emulation as per
>>>> the AHCI spec. Perhaps this is sufficient for the Xen layer, but someone
>>>> else will need to author the appropriate glue code.
>>>>
>>>> If "real" hot-plugging is not sufficient, we'll need to discuss further,
>>>> preferably over some RFC patches.
>>>
>>> That's fine. AHCI hot-plugging would go a long way and once we have
>>> that, the rest is easy.
>>
>> Can we get some more background on this?
>>
>> IIRC the IDE bits are needed to boot hvm guests, which goes like this:
>>
>>   (1) boot disk is hooked up using both xenbus and ide.
>>   (2) seabios boots using ide.
>>   (3) linux kernel activates xenbus, at which point qemu zaps the ide
>>       disks to avoid the disk being present twice in the system.
>>
>> Correct?
>>
>> Do we really want repeat this exercise for AHCI?  Alot has changed since
>> this boot hack for ide was added ...
>>
>> As far I know OVMF has xenbus drivers, so OVMF should already boot xen
>> guests just fine without this, correct?
> 
> I agree with you that the current unplug in nasty. Also I don't care
> much about AHCI, in fact I don't think we should be spending efforts
> into making that scenario work better. I think we should be working on
> OVMF instead and fix the bug about empty cdrom drives reported by Fabio.
> 

OVMF and AHCI go hand in hand here from my viewpoint. I'm happy to debug
any OVMF+SATA/AHCI problems that are reported.

Last I saw, Laszlo asked Fabio for some more information on this
problem, so I am waiting for that information to start work on that issue.

Thanks,
--js

> 
>> Can we just have xenbus drivers for seabios too?  seabios can run disk
>> drivers in 32bit mode meanwhile, so this should not be as difficult any
>> more as it used to be.
> 
> Sure, I would be happy to see that happen, but I won't be working on it.
>