Re: [PATCH 1/2][4.15?] tools/x86: don't rebuild cpuid-autogen.h every time

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.03.2021 10:59, Ian Jackson wrote:
> Jan Beulich writes ("[PATCH 1/2][4.15?] tools/x86: don't rebuild 
> cpuid-autogen.h every time"):
>> The first thing the "xen-dir" rule does is delete the entire xen/
>> subtree. Obviously this includes deleting xen/lib/x86/*autogen.h. As a
>> result there's no original version for $(move-if-changed ...) to compare
>> against, and hence the file and all its consumers would get rebuilt
>> every time. Introduce a "prep-y" rule to move xen/lib/x86/ on the side,
>> to then recover any *autogen.h from there prior to invoking the
>> respective recursive $(MAKE) invocation.
> 
> Urgh.  Thanks for working on this swamp.
> 
> However,
> 
>> +# Arrange for preserving of auto-generated headers (to avoid them getting
>> +# rebuilt every time): Move the entire xen/lib/x86/ to a temporary place.
>> +prep-$(CONFIG_X86):
>> +    rm -rf .xen-lib-x86
>> +    test ! -d xen/lib/x86 || mv xen/lib/x86 .xen-lib-x86
>> +
>>  all-$(CONFIG_X86): xen-dir
>> +    $(if $(wildcard .xen-lib-x86/*autogen.h),mv .xen-lib-x86/*autogen.h 
>> xen/lib/x86/)
>> +    rm -rf .xen-lib-x86
>>      $(MAKE) -C xen/lib/x86 all XEN_ROOT=$(XEN_ROOT) PYTHON=$(PYTHON)
> 
> Isn't there some better way of doing this ?  I am very wary of adding
> additional on-disk Makefile-managed state to a Makefile which is
> already going wrong.  I haven't thought about this in enough detail to
> identify a specific bug but I think convincing myself that it is
> definitely correct is nontrivial.
> 
> Perhaps we could do the removal with a find rune instead, so we can
> just skip the files we wanted to keep ?

Maybe, and I did consider the option, but it would have felt more
fragile to me than this dedicated keep-just-the-few-files approach.
The problems we've had with this symlinking don't make me confident
in leaving around parts of this subtree; populating from scratch
seems like the most robust model (short of the suggested but never
carried out removal of the symlinking) to me.

Jan