On Tue, Dec 9, 2025 at 1:49 AM Jan Beulich <[email protected]> wrote:
>
> On 09.12.2025 01:37, Saman Dehghan wrote:
> > 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 <[email protected]>
> > ---
> > xen/arch/arm/arch.mk | 1 +
> > xen/arch/arm/arm64/vfp.c | 6 ++++++
> > 2 files changed, 7 insertions(+)
>
> Please also update ./README then accordingly.
Thank you, Jan, I will do that.
~Saman
>
> > --- 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 )
>
> Aren't it save_state() and restore_state() which actually use FP registers?
> Applying such attributes in too wide a fashion risks the compiler using FP
> registers also for other purposes. On x86 we were hit by such when we
> suppressed use of SSE registers while not suppressing use of MMX ones. In
> some configuration, after many years of this having gone fine, the compiler
> chose to use MMX insns for some odd reason.
>
> Seeing that save_state() and restore_state() are a single asm() statement
> each, any reason not to actually have them in an assembly file, just like
> their SVE counterparts are?
>
On top of those, `READ_SYSREG(FPSR)`, `READ_SYSREG(FPCR)`,
`WRITE_SYSREG(v->arch.vfp.fpsr, FPSR)`and
`WRITE_SYSREG(v->arch.vfp.fpcr, FPCR)` using FP.
I think I cannot apply __attribute__ on statements.
~Saman
> Jan
