src/arch/x86/rdrand.c

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