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

[Vladimir Murzin <vladimir.murzin@arm.com>]

On 7/30/26 11:59, Mark Rutland wrote:
> 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.
> 

Awesome!

Cheers
Vladimir