Date: Wed, 13 Dec 2017 13:23:38 -0800
From: Stephen Hemminger <stephen@networkplumber.org>
To: Vincent Legoll <vincent.legoll@gmail.com>
Cc: kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com,
        devel@linuxdriverproject.org,
        Linux Kernel ML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] hyperv: make HYPERV a menuconfig to ease disabling it
 all
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On Wed, 13 Dec 2017 09:54:19 +0100
Vincent Legoll <vincent.legoll@gmail.com> wrote:

> Hello,
> 
> On Sun, Dec 10, 2017 at 6:50 AM, Stephen Hemminger
> <stephen@networkplumber.org> wrote:
> > Will this break existing configs?  
> 
> I don't think so. Last time I did some similar changes, the kbuild
> test robot found some warnings on some configurations, I hope
> it will find problems (if any) for that series too (this one is not alone,
> I've got a bunch of other similar patches in-flight)
> 
> Thanks

NAK

Let me give a concrete example of how this will break users.

1. Assume user has a working .config file in their kernel build directory
which builds a kernel that works on Hyper-V.

2. Add your patch (or assume it makes into a later version).

3. User then does

$ make oldconfig
scripts/kconfig/conf  --oldconfig Kconfig
*
* Restart config...
*
*
* Microsoft Hyper-V guest support
*
Microsoft Hyper-V guest support (HYPERV_MENU) [N/y] (NEW) 

If they hit return, the default value is not enabling HyperV and they
will then go on to build a kernel that will not boot on your system.

The default MUST be set to Yes.