Re: [RFC PATCH 06/13] arm64: percpu: Add infrastructure for preemptible this_cpu_*() ops

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Jul 29, 2026 at 03:25:52PM +0100, Vladimir Murzin wrote:
> On 7/28/26 13:38, Mark Rutland wrote:
> > +#define __VAL_PCPU_GPRS(pcp, off, addr)				\
> > +	"("							\
> > +		"(.L__gpr_num_" pcp  " << 0) | "		\
> > +		"(.L__gpr_num_" off  " << 5) | "		\
> > +		"(.L__gpr_num_" addr " << 10)"			\
> > +	")"
> 
> Later in the patch, there is a comment stating that these registers
> are not expected to overlap. I can also see that early-clobber
> constraints are applied to the output registers later in the patch
> series, so the code is correct.
> 
> However, would it be possible to add assertions that detect register
> overlap, perhaps something like:
> 
> ".if (.L__gpr_num_" pcp "== .L__gpr_num_" off ") ||"	\
> "    (.L__gpr_num_" pcp "== .L__gpr_num_" addr ") ||"	\
> "    (.L__gpr_num_" off "== .L__gpr_num_" addr")"	\
> ".error "inline asm registers overlap"			\
> ".endif"						\

That's a good idea. I've added the following:

| #define __ASSERT_PCPU_GPRS_UNIQUE(pcp, off, addr)                       \
|         ".if ("                                                         \
|                 "(.L__gpr_num_" pcp " == .L__gpr_num_" off ") || "      \
|                 "(.L__gpr_num_" pcp " == .L__gpr_num_" addr ") || "     \
|                 "(.L__gpr_num_" off " == .L__gpr_num_" addr")"          \
|         "    )\n"                                                       \
|         ".error \"PCPU GPRS overlap: {" pcp "," off "," addr "}\"\n"    \
|         ".endif\n"
| 
| #define ____PCPU_GPRS_BEGIN(gprs, pcp, off, addr)                       \
|         __DEFINE_ASM_GPR_NUMS                                           \
|         __DEFINE_ASM_GPR_ALIASES                                        \
|         __ASSERT_PCPU_GPRS_UNIQUE(pcp, off, addr)                       \
|         "       mov w" off ", #" __VAL_PCPU_GPRS(pcp, off, addr) "\n"   \
|         "       strh    w" off ", " gprs "\n"                           \
|         __KERN_ASM_CPU_OFFSET(off) "\n"

With that, we have enough ".L__gpr_num_" string concatenation that we
should probably have:

| #define __GPR_NUM(r)		"(.L__gpr_num_" r ")"

... so I'll go add a preparatory patches adding that and cleaning up
existing usage.

Mark.