[PATCH] xen/arm64: Add support Clang build on arm64

[Saman Dehghan]

This patch enables building Xen on arm64 architecture using the Clang compiler.
Changes include:
- Add explicit -march=armv8 flag for arm64 builds.
- Add `__attribute__((target("fp-armv8")))` to `vfp_save_state` and
  `vfp_restore_state` functions when building with Clang to allow
  FP instructions despite `-mgeneral-regs-only`.

Signed-off-by: Saman Dehghan <samaan.dehg...@gmail.com>
---
 xen/arch/arm/arch.mk     | 1 +
 xen/arch/arm/arm64/vfp.c | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/xen/arch/arm/arch.mk b/xen/arch/arm/arch.mk
index 9c4bedfb3b..bcf548069b 100644
--- a/xen/arch/arm/arch.mk
+++ b/xen/arch/arm/arch.mk
@@ -13,6 +13,7 @@ ifeq ($(CONFIG_MPU),y)
 CFLAGS-$(CONFIG_ARM_64) += -march=armv8-r
 else
 CFLAGS-$(CONFIG_ARM_64) += -mcpu=generic
+CFLAGS-$(CONFIG_ARM_64) += -march=armv8
 endif
 CFLAGS-$(CONFIG_ARM_64) += -mgeneral-regs-only # No fp registers etc
 $(call cc-option-add,CFLAGS-$(CONFIG_ARM_64),CC,-mno-outline-atomics)
diff --git a/xen/arch/arm/arm64/vfp.c b/xen/arch/arm/arm64/vfp.c
index c4f89c7b0e..51fd2ddc54 100644
--- a/xen/arch/arm/arm64/vfp.c
+++ b/xen/arch/arm/arm64/vfp.c
@@ -46,6 +46,9 @@ static inline void restore_state(const uint64_t *fpregs)
                  : : "Q" (*fpregs), "r" (fpregs));
 }
 
+#if defined(CONFIG_CC_IS_CLANG)
+__attribute__((target("fp-armv8")))
+#endif
 void vfp_save_state(struct vcpu *v)
 {
     if ( !cpu_has_fp )
@@ -62,6 +65,9 @@ void vfp_save_state(struct vcpu *v)
         v->arch.vfp.fpexc32_el2 = READ_SYSREG(FPEXC32_EL2);
 }
 
+#if defined(CONFIG_CC_IS_CLANG)
+__attribute__((target("fp-armv8")))
+#endif
 void vfp_restore_state(struct vcpu *v)
 {
     if ( !cpu_has_fp )
-- 
2.49.0