Re: [Qemu-block] [PATCH V2 3/5] virtio-blk: disable scsi passthrough by default

[Paolo Bonzini]

On 15/07/2015 16:28, Michael S. Tsirkin wrote:
> On Wed, Jul 15, 2015 at 04:18:49PM +0200, Paolo Bonzini wrote:
>>
>>
>> On 15/07/2015 16:14, Michael S. Tsirkin wrote:
>>> On Wed, Jul 15, 2015 at 02:47:24PM +0200, Paolo Bonzini wrote:
>>>>
>>>>
>>>> On 15/07/2015 14:21, Michael S. Tsirkin wrote:
>>>>>>> Disable scsi passthrough by default since it was incompatible with
>>>>>>> virtio 1.0. For legacy machine types, keep this on by default.
>>>>>>>
>>>>>>> Cc: Stefan Hajnoczi <address@hidden>
>>>>>>> Cc: Kevin Wolf <address@hidden>
>>>>>>> Cc: address@hidden
>>>>>>> Signed-off-by: Jason Wang <address@hidden>
>>>>> Seems risky for 2.4.  modern is off by default for now. Can't we limit
>>>>> the change to when modern is enabled?
>>>>
>>>> That would have the effect of disabling a feature when you turn on modern.
>>>
>>> What's wrong with that?
>>
>> Weren't you complaining about it a few hours ago? :)
> 
> No, I complained about guest driver update disabling it.

Ah, sorry for the confusion.

>>>>> I suggested changing this from bool to on/off/auto, and
>>>>> make auto mean !modern.
>>>>
>>>> No, please do it like Jason did.  The SCSI feature effectively had to be
>>>> enabled explicitly already, the requests were marked as unsupported.
>>>
>>> I didn't know. How is it enabled?
>>
>> It's enabled by default in QEMU, but disabled by default in libvirt.
>> And it only works if you pass a whole _disk_ (not a partition or logical
>> volume) to QEMU, which is definitely not the common case.
>>
>> It can just be documented in the release notes; the feature is still
>> available, and libvirt won't be broken because it adds explicitly both
>> scsi=on and scsi=off.
> 
> So for libvirt, we don't really care about the default, right?
> For command line, would it not be friendlier to make it follow the
> modern flag automatically?

I wouldn't mind if we grabbed the occasion to disable it altogether.
However, that would indeed work as well.

Paolo