linux-kernel - Re: [RFC PATCH 04/18] virt/mshv: request version ioctl

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Message-ID: <87eeguc61d.fsf@vitty.brq.redhat.com>
Date:   Fri, 05 Mar 2021 10:18:38 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
        linux-hyperv@...r.kernel.org
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, mikelley@...rosoft.com,
        viremana@...ux.microsoft.com, sunilmut@...rosoft.com,
        wei.liu@...nel.org, ligrassi@...rosoft.com, kys@...rosoft.com
Subject: Re: [RFC PATCH 04/18] virt/mshv: request version ioctl

Nuno Das Neves <nunodasneves@...ux.microsoft.com> writes:

> On 2/9/2021 5:11 AM, Vitaly Kuznetsov wrote:
>> Nuno Das Neves <nunodasneves@...ux.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