Nuno Das Neves [off-list ref] writes:

On 2/9/2021 5:11 AM, Vitaly Kuznetsov wrote:

quoted

Nuno Das Neves [off-list ref] writes:

...

quoted

quoted

+
+3.1 MSHV_REQUEST_VERSION
+------------------------
+:Type: /dev/mshv ioctl
+:Parameters: pointer to a u32
+:Returns: 0 on success
+
+Before issuing any other ioctls, a MSHV_REQUEST_VERSION ioctl must be called to
+establish the interface version with the kernel module.
+
+The caller should pass the MSHV_VERSION as an argument.
+
+The kernel module will check which interface versions it supports and return 0
+if one of them matches.
+
+This /dev/mshv file descriptor will remain 'locked' to that version as long as
+it is open - this ioctl can only be called once per open.
+

KVM used to have KVM_GET_API_VERSION too but this turned out to be not
very convenient so we use capabilities (KVM_CHECK_EXTENSION/KVM_ENABLE_CAP)
instead.

The goal of MSHV_REQUEST_VERSION is to support changes to APIs in the core set.
When we add new features/ioctls beyond the core we can use an extension/capability
approach like KVM.

Driver versions is a very bad idea from distribution/stable kernel point
of view as it presumes that the history is linear. It is not.

Imagine you have the following history upstream:

MSHV_REQUEST_VERSION = 1
<100 commits with features/fixes>
MSHV_REQUEST_VERSION = 2
<another 100 commits with features/fixes>
MSHV_REQUEST_VERSION = 2

Now I'm a linux distribution / stable kernel maintainer. My kernel is at
MSHV_REQUEST_VERSION = 1. Now I want to backport 1 feature from between
VER=1 and VER=2 and another feature from between VER=2 and VER=3. My
history now looks like

MSHV_REQUEST_VERSION = 1
<5 commits from between VER=1 and VER=2>
   Which version should I declare here???? 
<5 commits from between VER=2 and VER=3>
   Which version should I declare here???? 

If I keep VER=1 then userspace will think that I don't have any extra
features added and just won't use them. If I change VER to 2/3, it'll
think I have *all* features from between these versions.

The only reasonable way to manage this is to attach a "capability" to
every ABI change and expose this capability *in the same commit which
introduces the change to the ABI*. This way userspace will now exactly
which ioctls are available and what are their interfaces.

Also, trying to define "core set" is hard but you don't really need
to.

-- 
Vitaly