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Date:   Thu, 3 Dec 2020 14:38:07 +0100
From:   Stefano Garzarella <>
To:     "Paraschiv, Andra-Irina" <>,
        Stefan Hajnoczi <>
Cc:     netdev <>,
        linux-kernel <>,
        "David S . Miller" <>,
        David Duncan <>,
        Dexuan Cui <>,
        Alexander Graf <>,
        Jorgen Hansen <>,
        Jakub Kicinski <>,
        Vitaly Kuznetsov <>
Subject: Re: [PATCH net-next v1 1/3] vm_sockets: Include flag field in the
 vsock address data structure

On Thu, Dec 03, 2020 at 12:32:08PM +0200, Paraschiv, Andra-Irina wrote:
>On 03/12/2020 11:21, Stefan Hajnoczi wrote:
>>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 <>
>>>  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.
>>>+ */
>>>+/* 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.
>>>+ */
>>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.

I agree, VMADDR_FLAG_TO_HOST is more general and it's clearer that is up 
to the host where to forward the packet (sibling if supported, or 


>Thanks for the feedback, Stefan.
>I can update the naming to be more general, such asĀ  "_TO_HOST", and 
>keep the use cases (e.g. guest <-> host / nested / sibling VMs 
>communication) mention in the comments so that would relate more to 
>the motivation behind it.
>>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.
>Amazon Development Center (Romania) S.R.L. registered office: 27A Sf. Lazar Street, UBC5, floor 2, Iasi, Iasi County, 700045, Romania. Registered in Romania. Registration number J22/2621/2005.

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