On Tue, Dec 01, 2020 at 05:25:03PM +0200, Andra Paraschiv wrote:
> vsock enables communication between virtual machines and the host they
> are running on. With the multi transport support (guest->host and
> host->guest), nested VMs can also use vsock channels for communication.
> 
> In addition to this, by default, all the vsock packets are forwarded to
> the host, if no host->guest transport is loaded. This behavior can be
> implicitly used for enabling vsock communication between sibling VMs.
> 
> Add a flag field in the vsock address data structure that can be used to
> explicitly mark the vsock connection as being targeted for a certain
> type of communication. This way, can distinguish between nested VMs and
> sibling VMs use cases and can also setup them at the same time. Till
> now, could either have nested VMs or sibling VMs at a time using the
> vsock communication stack.
> 
> Use the already available "svm_reserved1" field and mark it as a flag
> field instead. This flag can be set when initializing the vsock address
> variable used for the connect() call.
> 
> Signed-off-by: Andra Paraschiv <andraprs@amazon.com>
> ---
>  include/uapi/linux/vm_sockets.h | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/vm_sockets.h b/include/uapi/linux/vm_sockets.h
> index fd0ed7221645d..58da5a91413ac 100644
> --- a/include/uapi/linux/vm_sockets.h
> +++ b/include/uapi/linux/vm_sockets.h
> @@ -114,6 +114,22 @@
>  
>  #define VMADDR_CID_HOST 2
>  
> +/* This sockaddr_vm flag value covers the current default use case:
> + * local vsock communication between guest and host and nested VMs setup.
> + * In addition to this, implicitly, the vsock packets are forwarded to the host
> + * if no host->guest vsock transport is set.
> + */
> +#define VMADDR_FLAG_DEFAULT_COMMUNICATION	0x0000
> +
> +/* Set this flag value in the sockaddr_vm corresponding field if the vsock
> + * channel needs to be setup between two sibling VMs running on the same host.
> + * This way can explicitly distinguish between vsock channels created for nested
> + * VMs (or local communication between guest and host) and the ones created for
> + * sibling VMs. And vsock channels for multiple use cases (nested / sibling VMs)
> + * can be setup at the same time.
> + */
> +#define VMADDR_FLAG_SIBLING_VMS_COMMUNICATION	0x0001

vsock has the h2g and g2h concept. It would be more general to call this
flag VMADDR_FLAG_G2H or less cryptically VMADDR_FLAG_TO_HOST.

That way it just tells the driver in which direction to send packets
without implying that sibling communication is possible (it's not
allowed by default on any transport).

I don't have a strong opinion on this but wanted to suggest the idea.

Stefan