Re: [PATCH 3/5] build: arm64: provide -target and -march if using clang

[Julien Grall <julien@xxxxxxx>]

Hi,

On 29/11/2024 22:12, Volodymyr Babchuk wrote:
> Hi Jan,
>
> Jan Beulich <jbeulich@xxxxxxxx> writes:
>
> > On 29.11.2024 02:49, Volodymyr Babchuk wrote:
> > > Provide -target and -march explicitly when building with clang. This
> > > makes cross-compilation much easier, because clang accept this
> > > parameters regardless of host platform. Basically,
> > >
> > >    make XEN_TARGET_ARCH=arm64 clang=y llvm=y
> > >
> > > will behave in the same way if building Xen on x86, or on arm64 or on
> > > any other platform.
> > >
> > > -march is required because with default value, clang will not
> > > recognize EL2 registers.
> > >
> > > Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
> > > ---
> > >   config/arm64.mk | 4 ++++
> > >   1 file changed, 4 insertions(+)
> > >
> > > diff --git a/config/arm64.mk b/config/arm64.mk
> > > index c4662f67d0..97eb9a82e7 100644
> > > --- a/config/arm64.mk
> > > +++ b/config/arm64.mk
> > > @@ -5,6 +5,10 @@ CONFIG_XEN_INSTALL_SUFFIX :=
> > >   
> > >   CFLAGS += #-marm -march= -mcpu= etc
> > >   
> > > +ifeq ($(clang),y)
> > > +CFLAGS += -target aarch64 -march=armv8-a
> > > +endif
> > Why is this dependent on (just?) $(clang), not (also?) $(llvm)?
> Because this parameter is handled by clang only. There is no harm in
> providing it explicitly. When building on arm64, value of this parameter
> will match the default value for the platform. When building on x86, we
> need to tell clang that it should generate arm64 code anyways. There is
> no reason in trying to make ARM build with x86 instruction set.
>
> > Also
> > this affects both toolstack builds and hypervisor. Is applying -march
> > like this actually appropriate for the toolstack?
> This is a good question. I can't see why this can't be appropriate for
> toolstack. I.e. what bad can happen when building the toolstack.
In the future, we may want to build the tools for Armv8-M. So I think 
the -march should also applies for the toolstack.
Cheers,

--
Julien Grall