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assemblyarminline-assemblyarm64msr

How to read the content of ID_AA64MMFR1_EL1 register into the 64-bit variable using inline assembly?

This is a follow-up question for this answer.

I'm trying to read the content of ID_AA64MMFR1_EL1 register into the 64-bit variable using inline assembly:

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>

int main(void)
{
    uint64_t foo;
    __asm volatile("mov %0, ID_AA64MMFR1_EL1" : "=r"(foo) ::);
    printf("%"PRIx64"\n", foo);
}

Note: I need to read ID_AA64MMFR1_EL1 in order to know the value of AFP field.

However, assembler rejects this code:

$ gcc t1b.c
/tmp/ccTDGH7d.s: Assembler messages:
/tmp/ccTDGH7d.s:22: Error: undefined symbol ID_AA64MMFR1_EL1 used as an immediate value

A simple question: how to do it correctly?

Solution

  • If your program is running at EL0, you should rather use the HWCAP_CPUID API available in hwcaps - more details in this article, ARM64 CPU Feature Registers. See here for a working example code.