src/arch/x86/rdrand.c

/* SPDX-License-Identifier: GPL-2.0-only */

#include <random.h>

/*
 * Intel recommends that applications attempt 10 retries in a tight loop
 * in the unlikely event that the RDRAND instruction does not successfully
 * return a random number. The odds of ten failures in a row would in fact
 * be an indication of a larger CPU issue.
 */
#define RDRAND_RETRY_LOOPS 10

/*
 * Generate a 32-bit random number through RDRAND instruction.
 * Carry flag is set on RDRAND success and 0 on failure.
 */
static inline uint8_t rdrand_32(uint32_t *rand)
{
	uint8_t carry;

	__asm__ __volatile__(
	     ".byte 0x0f; .byte 0xc7; .byte 0xf0; setc %1"
	     : "=a" (*rand), "=qm" (carry));
	return carry;
}

#if ENV_X86_64
/*
 * Generate a 64-bit random number through RDRAND instruction.
 * Carry flag is set on RDRAND success and 0 on failure.
 */
static inline uint8_t rdrand_64(uint64_t *rand)
{
	uint8_t carry;

	__asm__ __volatile__(
	     ".byte 0x48; .byte 0x0f; .byte 0xc7; .byte 0xf0; setc %1"
	     : "=a" (*rand), "=qm" (carry));
	return carry;
}
#endif

int get_random_number_32(uint32_t *rand)
{
	int i;

	/* Perform a loop call until RDRAND succeeds or returns failure. */
	for (i = 0; i < RDRAND_RETRY_LOOPS; i++) {
		if (rdrand_32(rand))
			return 0;
	}
	return -1;
}

int get_random_number_64(uint64_t *rand)
{
	int i;
	uint32_t rand_high, rand_low;

	/* Perform a loop call until RDRAND succeeds or returns failure. */
	for (i = 0; i < RDRAND_RETRY_LOOPS; i++) {
#if ENV_X86_64
		if (rdrand_64(rand))
			return 0;
#endif
		if (rdrand_32(&rand_high) && rdrand_32(&rand_low)) {
			*rand = ((uint64_t)rand_high << 32) |
				(uint64_t)rand_low;
			return 0;
		}
	}
	return -1;
}