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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-10-16 at 12:49 +0200, Markus Armbruster wrote:
> Ian Campbell <Ian.Campbell@xxxxxxxxxx> writes:
> 
> > On Wed, 2013-10-16 at 10:54 +0100, Jan Beulich wrote:
> >> >>> On 16.10.13 at 08:30, "Gonglei (Arei)" <arei.gonglei@xxxxxxxxxx> 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 $(call move-if-changed,$@.tmp,$@). 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

I think this is somewhat orthogonal to the use of move-if-changed which
is there to ensure that the timestamp doesn't change gratuitously and
cause a load of knock on rebuilds even though the regenerated content is
identical.

Ian.


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