Nuno Das Neves <nunodasneves at linux.microsoft.com> writes:
> On 2/9/2021 5:11 AM, Vitaly Kuznetsov wrote:
>> Nuno Das Neves <nunodasneves at linux.microsoft.com> writes:
>>
...>>> +
>>> +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