Re: [Qemu-devel] [Xen-devel] Hvmloader: Modify ACPI to only supply _EJ0 methods for PCIslots that support hotplug by runtime patching

[Markus Armbruster]

Ian Campbell <address@hidden> writes:

> On Wed, 2013-10-16 at 10:54 +0100, Jan Beulich wrote:
>> >>> On 16.10.13 at 08:30, "Gonglei (Arei)" <address@hidden> wrote:
>> > --- hvmloader/acpi/Makefile        2013-10-16 11:51:53.000000000 +0800
>> > +++ hvmloader_new//acpi/Makefile   2013-10-16 11:51:58.000000000 +0800
>> > @@ -36,18 +36,34 @@
>> >  
>> >  dsdt_anycpu_qemu_xen.asl: dsdt.asl mk_dsdt
>> >    awk 'NR > 1 {print s} {s=$$0}' $< > $@
>> > -  ./mk_dsdt --dm-version qemu-xen >> $@
>> > +  sed -i 's/AmlCode/dsdt_anycpu_qemu_xen/g' $@
>> 
>> This must never be done - if someone hits Ctrl-C in the middle of
>> this, you'll have a modified but incomplete generated file. You
>> either need to use properly chained rules, or do all output to a
>> temporary file which you rename as the last step.
>> 
>> I realize that the problem existed before your change, but you
>> making it worse requires doing it properly now.
>
> The correct way to do this is to generate to a temporary file and then
> use $(call move-if-changed,address@hidden,$@). There are a bunch of examples 
> in
> tools/libxl/Makefile and elsewhere.

Quoting the GNU make manual[*]:

    So generally the right thing to do is to delete the target file if
    the recipe fails after beginning to change the file.  make will do
    this if .DELETE_ON_ERROR appears as a target.  This is almost always
    what you want make to do, but it is not historical practice; so for
    compatibility, you must explicitly request it.

In my opinion, every Makefile should request it.

[*] https://www.gnu.org/software/make/manual/html_node/Errors.html