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Date:   Thu, 3 Dec 2020 12:32:08 +0200
From:   "Paraschiv, Andra-Irina" <>
To:     Stefan Hajnoczi <>
CC:     netdev <>,
        linux-kernel <>,
        "David S . Miller" <>,
        David Duncan <>,
        Dexuan Cui <>,
        Alexander Graf <>,
        Jorgen Hansen <>,
        Jakub Kicinski <>,
        Stefano Garzarella <>,
        Vitaly Kuznetsov <>
Subject: Re: [PATCH net-next v1 1/3] vm_sockets: Include flag field in the
 vsock address data structure

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.

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.
> Stefan

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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