Re: [PATCH] virtio-blk: drop deprecated scsi=on|off property

[Markus Armbruster]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Thu, Apr 29, 2021 at 04:52:21PM +0100, Stefan Hajnoczi wrote:
>> The scsi=on|off property was deprecated in QEMU 5.0 and can be removed
>> completely at this point.
>> 
>> Drop the scsi=on|off option. It was only available on Legacy virtio-blk
>> devices. Linux v5.6 already dropped support for it.
>> 
>> Remove the hw_compat_2_4[] property assignment since scsi=on|off no
>> longer exists. Old guests with Legacy virtio-blk devices no longer see
>> the SCSI host features bit.
>> 
>
> This means pc-2.4 will now break guest ABI if using virtio-blk
> devices, correct?
>
> This looks like a sign we should have deprecated pc-2.4 a long
> time ago.

The last batch of PC machine type retiring was pc-1.0 to pc-1.3:
deprecated in 5.0 (commit 30d2a17b4, Dec 2019), dropped in 6.0 (commit
f862ddbb1, just weeks ago).  pc-1.3 was a bit over seven years old when
we released 5.0.  pc-2.4 will be six years old by the time we release
6.1.  Fair game?

>> Live migrating old guests from an old QEMU with the SCSI feature bit
>> enabled will fail with "Features 0x... unsupported. Allowed features:
>> 0x...". We've followed the QEMU deprecation policy so users have been
>> warned...
>> 
>
> Were they really warned, though?  People running
> "-machine pc-i440fx-2.4" might be completely unaware that it was
> silently enabling a deprecated feature.

We've gotten better at documenting deprecations, but we're still bad at
warning on use of deprecated features.

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