linux-kernel - Re: [PATCH v2 4/4] kbuild: use $(src) instead of $(srctree)/$(src) for source directory

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Date: Fri, 28 Jun 2024 22:54:34 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "masahiroy@...nel.org" <masahiroy@...nel.org>
CC: "nicolas@...sle.eu" <nicolas@...sle.eu>, "conor@...nel.org"
	<conor@...nel.org>, "devicetree@...r.kernel.org"
	<devicetree@...r.kernel.org>, "robh@...nel.org" <robh@...nel.org>,
	"linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"krzk@...nel.org" <krzk@...nel.org>
Subject: Re: [PATCH v2 4/4] kbuild: use $(src) instead of $(srctree)/$(src)
 for source directory

On Fri, 2024-06-28 at 14:23 -0700, Rick Edgecombe wrote:
> On Fri, 2024-06-28 at 11:13 +0900, Masahiro Yamada wrote:
> > There are two solutions, depending on what you want to achieve.
> > 
> > The official way is to pass the absolute path to M=
> > (or relative path to ../linux-tdm-kvm-out)
> > 
> > The other unofficial way is to pass VPATH.
> > The external module build does not officially support
> > the separate output directory, but you can still
> > do it in this case.
> > 
> > [1] will work like before.
> 
> The absolute path worked, but why not make it use the relative path by default
> in this case? "arch/x86/kvm/" shouldn't be confused with an absolute path...

Argh, I missed that make ARCH=x86_64 O=../linux-tdx-kvm-out/ -j36 M=$(realpath
arch/x86/kvm) will output the the source tree instead of the output directory.

So there is no official way to build just a module to the output directory? And
it just accidentally worked all these years? IMO it's a nice feature to have.
I've especially found it useful when doing virtualization development where you
need to build/test guest and host kernels as the same time.

Or was it just now delegated to unofficial support after this patch?