Re: [PATCH v2 1/4] x86: provide an inverted Kconfig control for shim-exclusive mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.01.2025 10:49, Roger Pau Monné wrote:
> On Wed, Jan 22, 2025 at 09:43:53AM +0100, Jan Beulich wrote:
>> On 21.01.2025 19:02, Roger Pau Monné wrote:
>>> On Tue, Jan 21, 2025 at 11:35:42AM +0100, Jan Beulich wrote:
>>>> On 21.01.2025 09:52, Roger Pau Monné wrote:
>>>>> On Tue, Jan 21, 2025 at 09:13:38AM +0100, Jan Beulich wrote:
>>>>>> On 21.01.2025 00:27, Stefano Stabellini wrote:
>>>>>>> If I understood it right, I like Andrew's suggestion. He is suggesting
>>>>>>> to do the following:
>>>>>>>
>>>>>>> - turning PV_SHIM_EXCLUSIVE into something that does nothing
>>>>>>
>>>>>> FTAOD - you mean Kconfig-wise? Andrew clearly didn't say "nothing", but
>>>>>> "nothing other than making the boolean be a compile time constant".
>>>>>
>>>>> Won't making the boolean a compile time constant would also result in
>>>>> DCO kicking in and removing a fair amount of code?  So even if you
>>>>> have enabled everything in Kconfig, the resulting hypervisor would
>>>>> only be suitable to be used as a shim?
>>>>
>>>> Of course.
>>>
>>> Then what's the point of this approach?  Options will be enabled in
>>> Kconfig, but the resulting hypervisor build when using allyesconfig
>>> would have a lot of them short-circuited, making it even worse than
>>> currently?  As options will get effectively build-time disabled due
>>> to DCO while enabled in Kconfig.
>>
>> Well, I have to direct this question to Andrew. It is specifically
>> what I'm trying to address with UNCONSTRAINED.
>>
>>> Overall I think PV_SHIM_EXCLUSIVE should be excluded from
>>> allyesconfig, even with Andrew's proposed change.  Otherwise the
>>> purpose of allyesconfig is defeated if enabling PV_SHIM_EXCLUSIVE
>>> makes the resulting hypervisor build PV shim only.  IIRC we can
>>> provide a default alllyes.config with CONFIG_PV_SHIM_EXCLUSIVE=n.
>>
>> Hmm, I wasn't aware of the option of using allyes.config. That might be
>> the route to go, albeit it looks like people using the allyesconfig
>> target then need to remember to set KCONFIG_ALLCONFIG in the environment
>> (to <empty> or 1), or to explicitly specify a file name that way. (This
>> of course ought to be easy enough to arrange for in our automation.)
> 
> My knowledge of Kconfig is very limited, but isn't there a default
> path for such file to be picked up by Kconfig?  I see we already have
> a xen/tools/kconfig/allrandom.config, I was expecting it would be a
> matter of dropping an allyes.config in that directory, but I haven't
> tried.

Well, I simply looked at the kconfig sources, and my reading of it is
that it won't even try to open allyes.config when the envvar is absent.

Jan