src/arch/x86/rdrand.c

   1 /* SPDX-License-Identifier: GPL-2.0-only */
   2
   3 #include <random.h>
   4 #include <types.h>
   5
   6 /*
   7  * Intel recommends that applications attempt 10 retries in a tight loop
   8  * in the unlikely event that the RDRAND instruction does not successfully
   9  * return a random number. The odds of ten failures in a row would in fact
  10  * be an indication of a larger CPU issue.
  11  */
  12 #define RDRAND_RETRY_LOOPS 10
  13
  14 /*
  15  * Generate a 32-bit random number through RDRAND instruction.
  16  * Carry flag is set on RDRAND success and 0 on failure.
  17  */
  18 static inline uint8_t rdrand_32(uint32_t *rand)
  19 {
  20         uint8_t carry;
  21
  22         __asm__ __volatile__(
  23              ".byte 0x0f; .byte 0xc7; .byte 0xf0; setc %1"
  24              : "=a" (*rand), "=qm" (carry));
  25         return carry;
  26 }
  27
  28 #if ENV_X86_64
  29 /*
  30  * Generate a 64-bit random number through RDRAND instruction.
  31  * Carry flag is set on RDRAND success and 0 on failure.
  32  */
  33 static inline uint8_t rdrand_64(uint64_t *rand)
  34 {
  35         uint8_t carry;
  36
  37         __asm__ __volatile__(
  38              ".byte 0x48; .byte 0x0f; .byte 0xc7; .byte 0xf0; setc %1"
  39              : "=a" (*rand), "=qm" (carry));
  40         return carry;
  41 }
  42 #endif
  43
  44 enum cb_err get_random_number_32(uint32_t *rand)
  45 {
  46         int i;
  47
  48         /* Perform a loop call until RDRAND succeeds or returns failure. */
  49         for (i = 0; i < RDRAND_RETRY_LOOPS; i++) {
  50                 if (rdrand_32(rand))
  51                         return CB_SUCCESS;
  52         }
  53         return CB_ERR;
  54 }
  55
  56 enum cb_err get_random_number_64(uint64_t *rand)
  57 {
  58         int i;
  59         uint32_t rand_high, rand_low;
  60
  61         /* Perform a loop call until RDRAND succeeds or returns failure. */
  62         for (i = 0; i < RDRAND_RETRY_LOOPS; i++) {
  63 #if ENV_X86_64
  64                 if (rdrand_64(rand))
  65                         return CB_SUCCESS;
  66 #endif
  67                 if (rdrand_32(&rand_high) && rdrand_32(&rand_low)) {
  68                         *rand = ((uint64_t)rand_high << 32) |
  69                                 (uint64_t)rand_low;
  70                         return CB_SUCCESS;
  71                 }
  72         }
  73         return CB_ERR;
  74 }