Re: [PATCH 2/5] build: add possibility to use LLVM tools

["Anthony PERARD" <anthony.perard@xxxxxxxxxx>]

On Fri, Nov 29, 2024 at 08:57:47AM +0100, Jan Beulich wrote:
> On 29.11.2024 02:49, Volodymyr Babchuk wrote:
> > Currently, even if we are using clang as a C compiler, we still use
> > GNU binutils. This patch adds new option "llvm" that allows to use
> > linker, objcopy and all other tools from LLVM project. As LLVM tools
> > use different approach for cross-compilation, we don't need
> > CROSS_COMPILE prefix in this case.

It might be worth explaining what this different approach is ;-). My
guess is that you ask llvm/clang to build for a specific arch, via
XEN_TARGET_ARCH=riscv64, and llvm doesn't need different binaries will
just do what is needed, right? (with -march I guess).

> This new option is meant to control both toolstack and hypervisor builds?
> As to the latter, I assume you're aware we're trying to move away from
> this kind of command line control of the build.

Having "clang=y" is a bit useless when one can do "CC=clang" and let
the build system figure out what CC is. That's at least true for the
build system under "xen/".

But if one want to use the whole LLVM toolchain, it as to write a lot
more. To build Xen (just the hypervisor) with it, one would need to run:

make CC=clang CXX=clang++ AS=llvm-as LD=ld.lld LD_LTO=llvm-lto \
    ADDR2LINE=llvm-addr2line AR=llvm-ar RANLIB=llvm-ranlib NM=llvm-nm \
    STRIP=llvm-strip OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump \
    SIZEUTIL=llvm-size

So while it's possible, maybe introducing "llvm" or "LLVM" to switch
toolchain might be ok, to help. (Just because Linux did just that
recently:
    https://www.kernel.org/doc/html/latest/kbuild/llvm.html#the-llvm-argument
)

Beyond switching toolchain, I don't think $(llvm) or $(clang) should be
used in the build system, and instead rely on autodetection for
arguments. (There's already $(CONFIG_CC_IS_CLANG) and
$(CONFIG_LD_IS_LLVM) in the hypervisor's build system, via Kconfig)

At least for the hypervisor. For the toolstack, we should probably deal
with toolchain in ./configure.

Thanks,

-- 

Anthony Perard | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech