Subject: Re: [PATCH net-next v12 03/12] virtio: Expose generic device capability operations

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 24 Nov 2025 15:30:54 -0500

On Wed, Nov 19, 2025 at 01:15:14PM -0600, Daniel Jurgens wrote:
> Currently querying and setting capabilities is restricted to a single
> capability and contained within the virtio PCI driver. However, each
> device type has generic and device specific capabilities, that may be
> queried and set. In subsequent patches virtio_net will query and set
> flow filter capabilities.
> 
> This changes the size of virtio_admin_cmd_query_cap_id_result. It's safe
> to do because this data is written by DMA, so a newer controller can't
> overrun the size on an older kernel.
> 
> Signed-off-by: Daniel Jurgens <[email-masked]>
> Reviewed-by: Parav Pandit <[email-masked]>
> Reviewed-by: Xuan Zhuo <[email-masked]>
> 
> ---
> v4: Moved this logic from virtio_pci_modern to new file
>     virtio_admin_commands.
> 
> v12:
>   - Removed uapi virtio_pci include in virtio_admin.h. MST
>   - Added virtio_pci uapi include to virtio_admin_commands.c
>   - Put () around cap in macro. MST
>   - Removed nonsense comment above VIRTIO_ADMIN_MAX_CAP. MST
>   - +1 VIRTIO_ADMIN_MAX_CAP when calculating array size. MST
>   - Updated commit message
> ---
>  drivers/virtio/Makefile                |  2 +-
>  drivers/virtio/virtio_admin_commands.c | 91 ++++++++++++++++++++++++++
>  include/linux/virtio_admin.h           | 80 ++++++++++++++++++++++
>  include/uapi/linux/virtio_pci.h        |  6 +-
>  4 files changed, 176 insertions(+), 3 deletions(-)
>  create mode 100644 drivers/virtio/virtio_admin_commands.c
>  create mode 100644 include/linux/virtio_admin.h
> 
> diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile
> index eefcfe90d6b8..2b4a204dde33 100644
> --- a/drivers/virtio/Makefile
> +++ b/drivers/virtio/Makefile
> @@ -1,5 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0
> -obj-$(CONFIG_VIRTIO) += virtio.o virtio_ring.o
> +obj-$(CONFIG_VIRTIO) += virtio.o virtio_ring.o virtio_admin_commands.o
>  obj-$(CONFIG_VIRTIO_ANCHOR) += virtio_anchor.o
>  obj-$(CONFIG_VIRTIO_PCI_LIB) += virtio_pci_modern_dev.o
>  obj-$(CONFIG_VIRTIO_PCI_LIB_LEGACY) += virtio_pci_legacy_dev.o
> diff --git a/drivers/virtio/virtio_admin_commands.c b/drivers/virtio/virtio_admin_commands.c
> new file mode 100644
> index 000000000000..a2254e71e8dc
> --- /dev/null
> +++ b/drivers/virtio/virtio_admin_commands.c
> @@ -0,0 +1,91 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +#include <linux/virtio.h>
> +#include <linux/virtio_config.h>
> +#include <linux/virtio_admin.h>
> +#include <uapi/linux/virtio_pci.h>
> +
> +int virtio_admin_cap_id_list_query(struct virtio_device *vdev,
> +				   struct virtio_admin_cmd_query_cap_id_result *data)
> +{
> +	struct virtio_admin_cmd cmd = {};
> +	struct scatterlist result_sg;
> +
> +	if (!vdev->config->admin_cmd_exec)
> +		return -EOPNOTSUPP;
> +
> +	sg_init_one(&result_sg, data, sizeof(*data));
> +	cmd.opcode = cpu_to_le16(VIRTIO_ADMIN_CMD_CAP_ID_LIST_QUERY);
> +	cmd.group_type = cpu_to_le16(VIRTIO_ADMIN_GROUP_TYPE_SELF);
> +	cmd.result_sg = &result_sg;
> +
> +	return vdev->config->admin_cmd_exec(vdev, &cmd);
> +}
> +EXPORT_SYMBOL_GPL(virtio_admin_cap_id_list_query);
> +
> +int virtio_admin_cap_get(struct virtio_device *vdev,
> +			 u16 id,
> +			 void *caps,
> +			 size_t cap_size)

I still don't get why cap_size needs to be as large as size_t.

if you don't care what's it size is, just say "unsigned".
or u8 as a hint to users it's a small value.

> +{
> +	struct virtio_admin_cmd_cap_get_data *data;
> +	struct virtio_admin_cmd cmd = {};
> +	struct scatterlist result_sg;
> +	struct scatterlist data_sg;
> +	int err;
> +
> +	if (!vdev->config->admin_cmd_exec)
> +		return -EOPNOTSUPP;
> +
> +	data = kzalloc(sizeof(*data), GFP_KERNEL);

uses kzalloc without including linux/slab.h

> +	if (!data)
> +		return -ENOMEM;
> +
> +	data->id = cpu_to_le16(id);
> +	sg_init_one(&data_sg, data, sizeof(*data));
> +	sg_init_one(&result_sg, caps, cap_size);
> +	cmd.opcode = cpu_to_le16(VIRTIO_ADMIN_CMD_DEVICE_CAP_GET);
> +	cmd.group_type = cpu_to_le16(VIRTIO_ADMIN_GROUP_TYPE_SELF);
> +	cmd.data_sg = &data_sg;
> +	cmd.result_sg = &result_sg;
> +
> +	err = vdev->config->admin_cmd_exec(vdev, &cmd);
> +	kfree(data);
> +
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(virtio_admin_cap_get);
> +
> +int virtio_admin_cap_set(struct virtio_device *vdev,
> +			 u16 id,
> +			 const void *caps,
> +			 size_t cap_size)
> +{
> +	struct virtio_admin_cmd_cap_set_data *data;
> +	struct virtio_admin_cmd cmd = {};
> +	struct scatterlist data_sg;
> +	size_t data_size;
> +	int err;
> +
> +	if (!vdev->config->admin_cmd_exec)
> +		return -EOPNOTSUPP;
> +
> +	data_size = sizeof(*data) + cap_size;
> +	data = kzalloc(data_size, GFP_KERNEL);
> +	if (!data)
> +		return -ENOMEM;
> +
> +	data->id = cpu_to_le16(id);
> +	memcpy(data->cap_specific_data, caps, cap_size);
> +	sg_init_one(&data_sg, data, data_size);
> +	cmd.opcode = cpu_to_le16(VIRTIO_ADMIN_CMD_DRIVER_CAP_SET);
> +	cmd.group_type = cpu_to_le16(VIRTIO_ADMIN_GROUP_TYPE_SELF);
> +	cmd.data_sg = &data_sg;
> +	cmd.result_sg = NULL;
> +
> +	err = vdev->config->admin_cmd_exec(vdev, &cmd);
> +	kfree(data);
> +
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(virtio_admin_cap_set);
> diff --git a/include/linux/virtio_admin.h b/include/linux/virtio_admin.h
> new file mode 100644
> index 000000000000..4ab84d53c924
> --- /dev/null
> +++ b/include/linux/virtio_admin.h
> @@ -0,0 +1,80 @@
> +/* SPDX-License-Identifier: GPL-2.0-only
> + *
> + * Header file for virtio admin operations
> + */
> +
> +#ifndef _LINUX_VIRTIO_ADMIN_H
> +#define _LINUX_VIRTIO_ADMIN_H
> +
> +struct virtio_device;
> +struct virtio_admin_cmd_query_cap_id_result;
> +
> +/**
> + * VIRTIO_CAP_IN_LIST - Check if a capability is supported in the capability list
> + * @cap_list: Pointer to capability list structure containing supported_caps array
> + * @cap: Capability ID to check
> + *
> + * The cap_list contains a supported_caps array of little-endian 64-bit integers
> + * where each bit represents a capability. Bit 0 of the first element represents
> + * capability ID 0, bit 1 represents capability ID 1, and so on.
> + *
> + * Return: 1 if capability is supported, 0 otherwise
> + */
> +#define VIRTIO_CAP_IN_LIST(cap_list, cap) \
> +	(!!(1 & (le64_to_cpu(cap_list->supported_caps[(cap) / 64]) >> (cap) % 64)))
> +
> +/**
> + * virtio_admin_cap_id_list_query - Query the list of available capability IDs
> + * @vdev: The virtio device to query
> + * @data: Pointer to result structure (must be heap allocated)
> + *
> + * This function queries the virtio device for the list of available capability
> + * IDs that can be used with virtio_admin_cap_get() and virtio_admin_cap_set().
> + * The result is stored in the provided data structure.
> + *
> + * Return: 0 on success, -EOPNOTSUPP if the device doesn't support admin
> + * operations or capability queries, or a negative error code on other failures.
> + */
> +int virtio_admin_cap_id_list_query(struct virtio_device *vdev,
> +				   struct virtio_admin_cmd_query_cap_id_result *data);
> +
> +/**
> + * virtio_admin_cap_get - Get capability data for a specific capability ID
> + * @vdev: The virtio device
> + * @id: Capability ID to retrieve
> + * @caps: Pointer to capability data structure (must be heap allocated)
> + * @cap_size: Size of the capability data structure
> + *
> + * This function retrieves a specific capability from the virtio device.
> + * The capability data is stored in the provided buffer. The caller must
> + * ensure the buffer is large enough to hold the capability data.
> + *
> + * Return: 0 on success, -EOPNOTSUPP if the device doesn't support admin
> + * operations or capability retrieval, or a negative error code on other failures.
> + */
> +int virtio_admin_cap_get(struct virtio_device *vdev,
> +			 u16 id,
> +			 void *caps,
> +			 size_t cap_size);
> +
> +/**
> + * virtio_admin_cap_set - Set capability data for a specific capability ID
> + * @vdev: The virtio device
> + * @id: Capability ID to set
> + * @caps: Pointer to capability data structure (must be heap allocated)
> + * @cap_size: Size of the capability data structure
> + *
> + * This function sets a specific capability on the virtio device.
> + * The capability data is read from the provided buffer and applied
> + * to the device. The device may validate the capability data before
> + * applying it.
> + *
> + * Return: 0 on success, -EOPNOTSUPP if the device doesn't support admin
> + * operations or capability setting, or a negative error code on other failures.
> + */
> +int virtio_admin_cap_set(struct virtio_device *vdev,
> +			 u16 id,
> +			 const void *caps,
> +			 size_t cap_size);
> +
> +#endif /* _LINUX_VIRTIO_ADMIN_H */
> diff --git a/include/uapi/linux/virtio_pci.h b/include/uapi/linux/virtio_pci.h
> index c691ac210ce2..2e35fd8d4a95 100644
> --- a/include/uapi/linux/virtio_pci.h
> +++ b/include/uapi/linux/virtio_pci.h
> @@ -315,15 +315,17 @@ struct virtio_admin_cmd_notify_info_result {
>  
>  #define VIRTIO_DEV_PARTS_CAP 0x0000
>  
> +#define VIRTIO_ADMIN_MAX_CAP 0x0fff
> +
>  struct virtio_dev_parts_cap {
>  	__u8 get_parts_resource_objects_limit;
>  	__u8 set_parts_resource_objects_limit;
>  };
>  
> -#define MAX_CAP_ID __KERNEL_DIV_ROUND_UP(VIRTIO_DEV_PARTS_CAP + 1, 64)
> +#define VIRTIO_ADMIN_CAP_ID_ARRAY_SIZE __KERNEL_DIV_ROUND_UP(VIRTIO_ADMIN_MAX_CAP + 1, 64)
>  
>  struct virtio_admin_cmd_query_cap_id_result {
> -	__le64 supported_caps[MAX_CAP_ID];
> +	__le64 supported_caps[VIRTIO_ADMIN_CAP_ID_ARRAY_SIZE];
>  };
>  
>  struct virtio_admin_cmd_cap_get_data {
> -- 
> 2.50.1