Support of building Xen with Clang/LLVM on Arm?

[Saman Dehghan <samaan.dehghan@xxxxxxxxx>]

Hi xen-devel,

When preparing and testing another of my patch for LLVM coverage [1], I
encountered a few problems with Clang/LLVM build on ARM 64 platforms.

The first two are clang errors.

I observe the following errors when building the Xen master branch
for ARM 64 with Clang 19 and 20. If I comment out 
"CFLAGS-$(CONFIG_ARM_64) += -mgeneral-regs-only" in "xen/arch/arm/arch.mk"
the error is gone but the build fails during linking (see the third). 

Error message:

arch/arm/arm64/vfp.c:9:18: error: instruction requires: fp-armv8  
    9 |     asm volatile("stp q0, q1, [%1, #16 * 0]\n\t"          
      |                  ^                                        
<inline asm>:1:2: note: instantiated into assembly here           
    1 |         stp q0, q1, [x8, #16 * 0]                        
      |         ^                                                 
arch/arm/arm64/vfp.c:9:46: error: instruction requires: fp-armv8                
                                                    
    9 |     asm volatile("stp q0, q1, [%1, #16 * 0]\n\t"          
      |                                              ^            
<inline asm>:2:2: note: instantiated into assembly here           
    2 |         stp q2, q3, [x8, #16 * 2]                         
      |         ^                                                 
arch/arm/arm64/vfp.c:10:46: error: instruction requires: fp-armv8
   10 |                  "stp q2, q3, [%1, #16 * 2]\n\t"          
      |                                              ^           
<inline asm>:3:2: note: instantiated into assembly here
    3 |         stp q4, q5, [x8, #16 * 4]
      |         ^
arch/arm/arm64/vfp.c:11:46: error: instruction requires: fp-armv8
   11 |                  "stp q4, q5, [%1, #16 * 4]\n\t"
      |                                              ^
<inline asm>:4:2: note: instantiated into assembly here
    4 |         stp q6, q7, [x8, #16 * 6]
      |         ^
arch/arm/arm64/vfp.c:12:46: error: instruction requires: fp-armv8
   12 |                  "stp q6, q7, [%1, #16 * 6]\n\t"
      |                                              ^
<inline asm>:5:2: note: instantiated into assembly here
    5 |         stp q8, q9, [x8, #16 * 8]
      |         ^
arch/arm/arm64/vfp.c:13:46: error: instruction requires: fp-armv8
   13 |                  "stp q8, q9, [%1, #16 * 8]\n\t"
      |                                              ^
<inline asm>:6:2: note: instantiated into assembly here
    6 |         stp q10, q11, [x8, #16 * 10]
      |         ^

Reprroduction steps:

$ git clone https://xenbits.xen.org/git-http/xen
$ cd xen
$ git checkout eff32008be0d2718d32d60245650ff6f88fb3d13
$ make -C xen menuconfig clang=y
$ make xen clang=y

Tested with the following setup:

- Clang version: Ubuntu clang version 19.1.1 (1ubuntu1~24.04.2)
- Host: Ubuntu 24.04.3 LTS /  aarch64

Second, for Clang 18 or below there are another set of errors despite
commenting out the "-mgeneral-regs-only" flag:

arch/arm/arm64/mmu/head.S:288:13: error: expected writable system register or 
pstate
        msr TTBR0_EL2, x4
            ^
arch/arm/arm64/mmu/head.S:509:13: error: expected writable system register or 
pstate
        msr TTBR0_EL2, x0
            ^

Tested with:

- Clang version: Ubuntu clang version 18.1.3 (1ubuntu1)
- Host: Ubuntu 24.04.3 LTS /  aarch64

This works with GCC. It also works with Clang on x86 builds.

Third, if I specify "LD=ld.lld" with Clang 19 and 20 after commenting out
the "-mgeneral-regs-only" flag. I got this linking error:

ld.lld: error: common/device-tree/static-evtchn.init.o:(.rodata.str): offset is 
outside the section

As I read from the project README, under "C compiler and linker - For ARM",
only GCC is listed. So my general question is whether Xen supports building
with Clang/LLVM on ARM platforms.

Let me know if you need more details or a patch attempt.

[1] https://lists.xenproject.org/archives/html/xen-devel/2025-10/msg00010.html

Thanks,
Saman