Re: [PATCH v5 8/8] common: honor CONFIG_CC_SPLIT_SECTIONS also for assembly functions

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.01.2024 11:36, Roger Pau Monné wrote:
> On Mon, Jan 15, 2024 at 03:40:19PM +0100, Jan Beulich wrote:
>> Leverage the new infrastructure in xen/linkage.h to also switch to per-
>> function sections (when configured), deriving the specific name from the
>> "base" section in use at the time FUNC() is invoked.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> TBD: Since we use .subsection in UNLIKELY_START(), a perhaps not really
>>      wanted side effect of this change is that respective out-of-line
>>      code now moves much closer to its original (invoking) code.
> 
> Hm, I'm afraid I don't have much useful suggestions here.
> 
>> TBD: Of course something with the same overall effect, but less
>>      impactful might do in Config.mk. E.g. $(filter-out -D%,$(3))
>>      instead of $(firstword (3)).
> 
> I don't have a strong opinion re those two options  My preference
> however (see below for reasoning) would be to put this detection in
> Kconfig.
> 
>> TBD: On top of Roger's respective patch (for livepatch), also respect
>>      CONFIG_FUNCTION_ALIGNMENT.
> 
> I think you can drop that, as the series is blocked.

Considering the series here has been pending for quite some time, too,
I guess I'd like to keep it just in case that other functionality
becomes unblocked or available by some other means, even if only to
remind myself.

>> --- a/xen/Makefile
>> +++ b/xen/Makefile
>> @@ -409,6 +409,9 @@ AFLAGS += -D__ASSEMBLY__
>>  
>>  $(call cc-option-add,AFLAGS,CC,-Wa$$(comma)--noexecstack)
>>  
>> +# Check to see whether the assmbler supports the --sectname-subst option.
>> +$(call cc-option-add,AFLAGS,CC,-Wa$$(comma)--sectname-subst 
>> -DHAVE_AS_SECTNAME_SUBST)
> 
> I guess you already know what I'm going to comment on.  I think this
> would be clearer if it was a Kconfig option.  For once because I think
> we should gate livapatch support on the option being available, and
> secondly it would avoid having to pass the extra -D around.
> 
> I think it's relevant to have a consistent set of build tool options
> requirements for livepatch support, so that when enabled the support
> is consistent across builds.  With this approach livepatch could be
> enabled in Kconfig, but depending on the tools support the resulting
> binary might or might not support live patching of assembly code.
> Such behavior is IMO unhelpful from a user PoV, and can lead to
> surprises down the road.

I can see the desire to have LIVEPATCH grow such a dependency. Yet there
is the bigger still open topic of the criteria towards what, if anything,
to probe for in Kconfig, what, if anything, to probe for in Makefile, and
which of the probing perhaps do in both places. I'm afraid my attempts to
move us closer to a resolution (topic on summit, making proposals on
list) have utterly failed. IOW I don't currently see how the existing
disagreement there can be resolved, which will result in me to continue
following the (traditional) Makefile approach unless I clearly view
Kconfig superior in a particular case. I could perhaps be talked into
following a "mixed Kconfig/Makefile model", along the lines of "x86:
convert CET tool chain feature checks to mixed Kconfig/Makefile model",
albeit I'm sure you're aware there are issues to sort out there, which I
see no value in putting time into as long as I can't expect things to
make progress subsequently.

Dropping this patch, while an option, would seem undesirable to me, since
even without Kconfig probing using new enough tool chains the splitting
would yield reliable results, and provide - imo - an improvement over
what we have right now.

Jan