[PATCH v2 1/8] virtio: introduce VirtIOConfigSizeParams & virtio_get_config_size

[Daniil Tatianin]

This is the first step towards moving all device config size calculation
logic into the virtio core code. In particular, this adds a struct that
contains all the necessary information for common virtio code to be able
to calculate the final config size for a device. This is expected to be
used with the new virtio_get_config_size helper, which calculates the
final length based on the provided host features.

This builds on top of already existing code like VirtIOFeature and
virtio_feature_get_config_size(), but adds additional fields, as well as
sanity checking so that device-specifc code doesn't have to duplicate it.

An example usage would be:

    static const VirtIOFeature dev_features[] = {
        {.flags = 1ULL << FEATURE_1_BIT,
         .end = endof(struct virtio_dev_config, feature_1)},
        {.flags = 1ULL << FEATURE_2_BIT,
         .end = endof(struct virtio_dev_config, feature_2)},
        {}
    };

    static const VirtIOConfigSizeParams dev_cfg_size_params = {
        .min_size = DEV_BASE_CONFIG_SIZE,
        .max_size = sizeof(struct virtio_dev_config),
        .feature_sizes = dev_features
    };

    // code inside my_dev_device_realize()
    size_t config_size = virtio_get_config_size(&dev_cfg_size_params,
                                                host_features);
    virtio_init(vdev, VIRTIO_ID_MYDEV, config_size);

Currently every device is expected to write its own boilerplate from the
example above in device_realize(), however, the next step of this
transition is moving VirtIOConfigSizeParams into VirtioDeviceClass,
so that it can be done automatically by the virtio initialization code.

Signed-off-by: Daniil Tatianin <d-tatia...@yandex-team.ru>
---
 hw/virtio/virtio.c         | 17 +++++++++++++++++
 include/hw/virtio/virtio.h |  9 +++++++++
 2 files changed, 26 insertions(+)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 5d607aeaa0..8518382025 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -3014,6 +3014,23 @@ size_t virtio_feature_get_config_size(const 
VirtIOFeature *feature_sizes,
     return config_size;
 }
 
+size_t virtio_get_config_size(const VirtIOConfigSizeParams *params,
+                              uint64_t host_features)
+{
+    size_t config_size = params->min_size;
+    const VirtIOFeature *feature_sizes = params->feature_sizes;
+    size_t i;
+
+    for (i = 0; feature_sizes[i].flags != 0; i++) {
+        if (host_features & feature_sizes[i].flags) {
+            config_size = MAX(feature_sizes[i].end, config_size);
+        }
+    }
+
+    assert(config_size <= params->max_size);
+    return config_size;
+}
+
 int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
 {
     int i, ret;
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index db1c0ddf6b..1991c58d9b 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -44,6 +44,15 @@ typedef struct VirtIOFeature {
     size_t end;
 } VirtIOFeature;
 
+typedef struct VirtIOConfigSizeParams {
+    size_t min_size;
+    size_t max_size;
+    const VirtIOFeature *feature_sizes;
+} VirtIOConfigSizeParams;
+
+size_t virtio_get_config_size(const VirtIOConfigSizeParams *params,
+                              uint64_t host_features);
+
 size_t virtio_feature_get_config_size(const VirtIOFeature *features,
                                       uint64_t host_features);
 
-- 
2.25.1