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

[Paolo Bonzini]

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? :)

>>> 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.

Paolo