Re: [PATCH RFC v3 06/13] virtio-pci: implement support for extended features

[Paolo Abeni <pabeni@redhat.com>]

On 7/23/25 7:47 AM, Jason Wang wrote:
> On Tue, Jul 22, 2025 at 3:37 PM Paolo Abeni <pabeni@redhat.com> wrote:
>>
>> On 7/22/25 5:28 AM, Jason Wang wrote:
>>> On Fri, Jul 18, 2025 at 4:53 PM Paolo Abeni <pabeni@redhat.com> wrote:
>>>>
>>>> Extend the features configuration space to 128 bits, and allow the
>>>> common read/write operation to access all of it.
>>>>
>>>> On migration, save the 128 bit version of the features only if the
>>>> upper bits are non zero. Relay reset to clear all the feature
>>>> space before load.
>>>>
>>>> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
>>>
>>> This is a guest noticeable behaviour change. I wonder if we need a
>>> command line option to enable and disable this feature for migration
>>> compatibility.
>>
>> This point is not clear to me, could you please elaborate a bit more? do
>> you mean we need i.e. a DEFINE_PROP_BOOL() or the like to enable the 128
>> bit space usage?
> 
> Yes, or maybe have a way to let the device to enable it automatically
> E.g when UDP GSO is enabled for virtio-net.

I think we can safely enable the extended space access (read/write) if
virtio_features_use_ex(vdev->host_features_ex). That should cover also
any eventual future addition of more extended features - even of
different devices.

/P