virtio_pci: Optimize virtio_pci_device structure size

Improve the size of the virtio_pci_device structure, which is commonly
used to represent a virtio PCI device. A given virtio PCI device can
either of legacy type or modern type, with the
struct virtio_pci_legacy_device occupying 32 bytes and the
struct virtio_pci_modern_device occupying 88 bytes. Make them a union,
thereby save 32 bytes of memory as shown by the pahole tool. This
improvement is particularly beneficial when dealing with numerous
devices, as it helps conserve memory resources.

Before the modification, pahole tool reported the following:
struct virtio_pci_device {
[...]
        struct virtio_pci_legacy_device ldev;            /*   824    32 */
        /* --- cacheline 13 boundary (832 bytes) was 24 bytes ago --- */
        struct virtio_pci_modern_device mdev;            /*   856    88 */

        /* XXX last struct has 4 bytes of padding */
[...]
        /* size: 1056, cachelines: 17, members: 19 */
[...]
};

After the modification, pahole tool reported the following:
struct virtio_pci_device {
[...]
        union {
                struct virtio_pci_legacy_device ldev;    /*   824    32 */
                struct virtio_pci_modern_device mdev;    /*   824    88 */
        };                                               /*   824    88 */
[...]
	/* size: 1024, cachelines: 16, members: 18 */
[...]
};
Signed-off-by: Feng Liu <feliu@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20230516135446.16266-1-feliu@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/virtio/virtio_pci_common.h

@@ -45,9 +45,10 @@ struct virtio_pci_vq_info {
 struct virtio_pci_device {
 	struct virtio_device vdev;
 	struct pci_dev *pci_dev;
-	struct virtio_pci_legacy_device ldev;
-	struct virtio_pci_modern_device mdev;
-
+	union {
+		struct virtio_pci_legacy_device ldev;
+		struct virtio_pci_modern_device mdev;
+	};
 	bool is_legacy;
 
 	/* Where to read and clear interrupt */