Re: [PATCH net-next v1 1/3] vm_sockets: Include flag field in the vsock address data structure

From: Paraschiv, Andra-Irina
Date: Thu Dec 03 2020 - 05:33:29 EST




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 <andraprs@xxxxxxxxxx>
---
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.

Thanks for the feedback, Stefan.

I can update the naming to be more general, such as  "_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.

Andra


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




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