Re: [PATCH v5 4/4] virtio: Convert feature properties to OnOffAuto

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2025/02/21 0:46, Michael S. Tsirkin wrote:
> On Sat, Feb 08, 2025 at 04:51:10PM +0900, Akihiko Odaki wrote:
>> Some features are not always available with vhost. Legacy features are
>> not available with vp_vdpa in particular. virtio devices used to disable
>> them when not available even if the corresponding properties were
>> explicitly set to "on".
>>
>> QEMU already has OnOffAuto type, which includes the "auto" value to let
>> it automatically decide the effective value. Convert feature properties
>> to OnOffAuto and set them "auto" by default to utilize it. This allows
>> QEMU to report an error if they are set "on" and the corresponding
>> features are not available.
>>
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> 
>> ---
>>   include/hw/virtio/virtio.h | 38 +++++++++++++++++++++-----------------
>>   hw/core/machine.c          |  1 +
>>   hw/virtio/virtio-bus.c     | 14 ++++++++++++--
>>   hw/virtio/virtio.c         |  4 +++-
>>   4 files changed, 37 insertions(+), 20 deletions(-)
>>
>> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
>> index 638691028050d2599592d8c7e95c75ac3913fbdd..b854c2cb1d04da0a35165289c28f87e8cb869df6 100644
>> --- a/include/hw/virtio/virtio.h
>> +++ b/include/hw/virtio/virtio.h
>> @@ -113,7 +113,8 @@ struct VirtIODevice
>>       uint16_t queue_sel;
>>       /**
>>        * These fields represent a set of VirtIO features at various
>> -     * levels of the stack. @host_features indicates the complete
>> +     * levels of the stack. @requested_features indicates the feature
>> +     * set the user requested. @host_features indicates the complete
>>        * feature set the VirtIO device can offer to the driver.
>>        * @guest_features indicates which features the VirtIO driver has
>>        * selected by writing to the feature register. Finally
>> @@ -121,6 +122,7 @@ struct VirtIODevice
>>        * backend (e.g. vhost) and could potentially be a subset of the
>>        * total feature set offered by QEMU.
>>        */
>> +    OnOffAutoBit64 requested_features;
>>       uint64_t host_features;
>>       uint64_t guest_features;
>>       uint64_t backend_features;
>> @@ -149,6 +151,7 @@ struct VirtIODevice
>>       bool started;
>>       bool start_on_kick; /* when virtio 1.0 feature has not been negotiated */
>>       bool disable_legacy_check;
>> +    bool force_features_auto;
>>       bool vhost_started;
>>       VMChangeStateEntry *vmstate;
>>       char *bus_name;
>> @@ -376,22 +379,23 @@ typedef struct VirtIOSCSIConf VirtIOSCSIConf;
>>   typedef struct VirtIORNGConf VirtIORNGConf;
>>   
>>   #define DEFINE_VIRTIO_COMMON_FEATURES(_state, _field) \
>> -    DEFINE_PROP_BIT64("indirect_desc", _state, _field,    \
>> -                      VIRTIO_RING_F_INDIRECT_DESC, true), \
>> -    DEFINE_PROP_BIT64("event_idx", _state, _field,        \
>> -                      VIRTIO_RING_F_EVENT_IDX, true),     \
>> -    DEFINE_PROP_BIT64("notify_on_empty", _state, _field,  \
>> -                      VIRTIO_F_NOTIFY_ON_EMPTY, true), \
>> -    DEFINE_PROP_BIT64("any_layout", _state, _field, \
>> -                      VIRTIO_F_ANY_LAYOUT, true), \
>> -    DEFINE_PROP_BIT64("iommu_platform", _state, _field, \
>> -                      VIRTIO_F_IOMMU_PLATFORM, false), \
>> -    DEFINE_PROP_BIT64("packed", _state, _field, \
>> -                      VIRTIO_F_RING_PACKED, false), \
>> -    DEFINE_PROP_BIT64("queue_reset", _state, _field, \
>> -                      VIRTIO_F_RING_RESET, true), \
>> -    DEFINE_PROP_BIT64("in_order", _state, _field, \
>> -                      VIRTIO_F_IN_ORDER, false)
>> +    DEFINE_PROP_ON_OFF_AUTO_BIT64("indirect_desc", _state, _field, \
>> +                                  VIRTIO_RING_F_INDIRECT_DESC, \
>> +                                  ON_OFF_AUTO_AUTO), \
>> +    DEFINE_PROP_ON_OFF_AUTO_BIT64("event_idx", _state, _field, \
>> +                                  VIRTIO_RING_F_EVENT_IDX, ON_OFF_AUTO_AUTO), \
>> +    DEFINE_PROP_ON_OFF_AUTO_BIT64("notify_on_empty", _state, _field, \
>> +                                  VIRTIO_F_NOTIFY_ON_EMPTY, ON_OFF_AUTO_AUTO), \
>> +    DEFINE_PROP_ON_OFF_AUTO_BIT64("any_layout", _state, _field, \
>> +                                  VIRTIO_F_ANY_LAYOUT, ON_OFF_AUTO_AUTO), \
>> +    DEFINE_PROP_ON_OFF_AUTO_BIT64("iommu_platform", _state, _field, \
>> +                                  VIRTIO_F_IOMMU_PLATFORM, ON_OFF_AUTO_OFF), \
>> +    DEFINE_PROP_ON_OFF_AUTO_BIT64("packed", _state, _field, \
>> +                                  VIRTIO_F_RING_PACKED, ON_OFF_AUTO_OFF), \
>> +    DEFINE_PROP_ON_OFF_AUTO_BIT64("queue_reset", _state, _field, \
>> +                                  VIRTIO_F_RING_RESET, ON_OFF_AUTO_AUTO), \
>> +    DEFINE_PROP_ON_OFF_AUTO_BIT64("in_order", _state, _field, \
>> +                                  VIRTIO_F_IN_ORDER, ON_OFF_AUTO_OFF)
>>   
>>   hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n);
>>   bool virtio_queue_enabled_legacy(VirtIODevice *vdev, int n);
>> diff --git a/hw/core/machine.c b/hw/core/machine.c
>> index c23b39949649054ac59d2a9b497f34e1b7bd8d6c..0de04baa61735ff02f797f778c626ef690625ce3 100644
>> --- a/hw/core/machine.c
>> +++ b/hw/core/machine.c
>> @@ -38,6 +38,7 @@
>>   
>>   GlobalProperty hw_compat_9_2[] = {
>>       {"arm-cpu", "backcompat-pauth-default-use-qarma5", "true"},
>> +    { TYPE_VIRTIO_DEVICE, "x-force-features-auto", "on" },
>>   };
>>   const size_t hw_compat_9_2_len = G_N_ELEMENTS(hw_compat_9_2);
>>   
> 
> Confused why this is on.
> If set, this breaks existing setups if they run a configuration vhost
> does not support, does it not?
> And in particular, aren't some of these features exposed through
> libvirt?

This property enforces the existing behavior, which is to disable 
unsupported features even if they are explicitly set to "on". It 
introduces no breaking change.

> 
>> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
>> index 896feb37a1caa805543e971c150d3673675b9a6b..75d433b252d5337d91616a2847b3dc12e811c2da 100644
>> --- a/hw/virtio/virtio-bus.c
>> +++ b/hw/virtio/virtio-bus.c
>> @@ -50,6 +50,7 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
>>       bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
>>       bool vdev_has_iommu;
>>       Error *local_err = NULL;
>> +    uint64_t features;
>>   
>>       DPRINTF("%s: plug device.\n", qbus->name);
>>   
>> @@ -63,13 +64,22 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
>>   
>>       /* Get the features of the plugged device. */
>>       assert(vdc->get_features != NULL);
>> -    vdev->host_features = vdc->get_features(vdev, vdev->host_features,
>> -                                            &local_err);
>> +    features = vdev->host_features | vdev->requested_features.auto_bits |
>> +               vdev->requested_features.on_bits;
>> +    features = vdc->get_features(vdev, features, &local_err);
>>       if (local_err) {
>>           error_propagate(errp, local_err);
>>           return;
>>       }
>>   
>> +    if (!vdev->force_features_auto &&
>> +        (features & vdev->requested_features.on_bits) != vdev->requested_features.on_bits) {
>> +        error_setg(errp, "A requested feature is not supported by the device");
>> +        return;
>> +    }
>> +
>> +    vdev->host_features = features;
>> +
>>       if (klass->device_plugged != NULL) {
>>           klass->device_plugged(qbus->parent, &local_err);
>>       }
>> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
>> index 85110bce37443bb46c4159761af112d0dba466b4..83f803fc703da6257608e21476305c8e9c6a8b07 100644
>> --- a/hw/virtio/virtio.c
>> +++ b/hw/virtio/virtio.c
>> @@ -4013,11 +4013,13 @@ static void virtio_device_instance_finalize(Object *obj)
>>   }
>>   
>>   static const Property virtio_properties[] = {
>> -    DEFINE_VIRTIO_COMMON_FEATURES(VirtIODevice, host_features),
>> +    DEFINE_VIRTIO_COMMON_FEATURES(VirtIODevice, requested_features),
>>       DEFINE_PROP_BOOL("use-started", VirtIODevice, use_started, true),
>>       DEFINE_PROP_BOOL("use-disabled-flag", VirtIODevice, use_disabled_flag, true),
>>       DEFINE_PROP_BOOL("x-disable-legacy-check", VirtIODevice,
>>                        disable_legacy_check, false),
>> +    DEFINE_PROP_BOOL("x-force-features-auto", VirtIODevice,
>> +                     force_features_auto, false),
>>   };
>>   
>>   static int virtio_device_start_ioeventfd_impl(VirtIODevice *vdev)
>>
>> -- 
>> 2.48.1
>