> > + u32 __reserved; > > Knowing ~ nothing about virtio with ~ no experience of it I'll offer up > a question/statement anyway. > > Aren't your reservations here a bit conservative ? Would you not be well > advised to reserve a bit more space per other virtio protocols ? > > ➜ grep reserved ./include/uapi/linux/*.h | grep virtio > ./include/uapi/linux/virtio_balloon.h: * __u8 reserved[6]; > ./include/uapi/linux/virtio_balloon.h: * In other words, add explicit > reserved fields to align field and > ./include/uapi/linux/virtio_blk.h: __u8 reserved[38]; > ./include/uapi/linux/virtio_blk.h: __u8 reserved[56]; > ./include/uapi/linux/virtio_config.h: * VIRTIO_TRANSPORT_F_END are > reserved for the transport > ./include/uapi/linux/virtio_crypto.h: __le32 reserved; > ./include/uapi/linux/virtio_i2c.h: * Copyright (c) 2021 Intel > Corporation. All rights reserved. > ./include/uapi/linux/virtio_input.h: __u8 reserved[5]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[3]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[3]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[3]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[4]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[8]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[4]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[3]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[64]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved[3]; > ./include/uapi/linux/virtio_iommu.h: __u8 > reserved2[4]; > ./include/uapi/linux/virtio_net.h: __le16 reserved[4]; > ./include/uapi/linux/virtio_net.h: __le16 reserved; > ./include/uapi/linux/virtio_net.h: __le16 reserved[3]; > ./include/uapi/linux/virtio_net.h: __u8 reserved; > ./include/uapi/linux/virtio_net.h: __le16 reserved1; > ./include/uapi/linux/virtio_pcidev.h: * @VIRTIO_PCIDEV_OP_RESERVED: > reserved to catch errors > ./include/uapi/linux/virtio_pcidev.h: * @reserved: reserved > ./include/uapi/linux/virtio_pcidev.h: __u16 reserved; > ./include/uapi/linux/virtio_pci.h: * 2-65535 - reserved > ./include/uapi/linux/virtio_pci.h: /* Unused, reserved for future > extensions. */ > ./include/uapi/linux/virtio_pci.h: __u8 reserved1[12]; > ./include/uapi/linux/virtio_pci.h: /* Unused, reserved for future > extensions. */ > ./include/uapi/linux/virtio_pci.h: __u8 reserved2[4]; > ./include/uapi/linux/virtio_pci.h: __u8 reserved[7]; > ./include/uapi/linux/virtio_pci.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_pci.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_pci.h: __u8 reserved[2]; > ./include/uapi/linux/virtio_pci.h: __u8 reserved[7]; > ./include/uapi/linux/virtio_pci.h: __u8 reserved[7]; > ./include/uapi/linux/virtio_pci.h: __u8 reserved; > ./include/uapi/linux/virtio_pci.h: __le32 reserved; > ./include/uapi/linux/virtio_pci.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_pci.h: __le32 reserved; > ./include/uapi/linux/virtio_pci.h: __le32 reserved; > ./include/uapi/linux/virtio_pci.h: __le32 reserved; > ./include/uapi/linux/virtio_pci.h: __u8 reserved[7]; > ./include/uapi/linux/virtio_rtc.h: * Copyright (c) 2024 Qualcomm > Innovation Center, Inc. All rights reserved. > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[7]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[5]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[5]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[5]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[7]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[7]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[5]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[5]; > ./include/uapi/linux/virtio_rtc.h: __u8 reserved[6]; > ./include/uapi/linux/virtio_spi.h: * Copyright (C) 2025 Qualcomm > Innovation Center, Inc. All rights reserved. > ./include/uapi/linux/virtio_spi.h: * other bits are reserved as 0, > 1-bit transfer is always supported. > ./include/uapi/linux/virtio_spi.h: * other bits are reserved as 0, > 1-bit transfer is always supported. > ./include/uapi/linux/virtio_spi.h: * @reserved: for future use. > ./include/uapi/linux/virtio_spi.h: __u8 reserved[3];+ u32 > __reserved;
I'm not an expert here, but taking a look at those files, the vast majority of those reserved fields appear to be padding to ensure the struct has 64-bit alignment, which matches the use here in virtio-media as well. virtio_pci appears to be the only device that explicitly states the reserved bytes are for future extensions. Unless there's a good a reason to expect a future use case where more space is needed, I would prefer to not add more at this time.
