[rng] Allow entropy source to be selected at runtime

As noted in commit 3c83843 ("[rng] Check for several functioning RTC
interrupts"), experimentation shows that Hyper-V cannot be trusted to
reliably generate RTC interrupts.  (As noted in commit f3ba0fb
("[hyperv] Provide timer based on the 10MHz time reference count
MSR"), Hyper-V appears to suffer from a general problem in reliably
generating any legacy interrupts.)  An alternative entropy source is
therefore required for an image that may be used in a Hyper-V Gen1
virtual machine.

The x86 RDRAND instruction provides a suitable alternative entropy
source, but may not be supported by all CPUs.  We must therefore allow
for multiple entropy sources to be compiled in, with the single active
entropy source selected only at runtime.

Restructure the internal entropy API to allow a working entropy source
to be detected and chosen at runtime.

Enable the RDRAND entropy source for all x86 builds, since it is
likely to be substantially faster than any other source.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

4 files changed, 313 insertions, 222 deletions

diff --git a/src/include/ipxe/efi/efi_entropy.h b/src/include/ipxe/efi/efi_entropy.h
deleted file mode 100644
index 5b16fd7f..00000000
--- a/src/include/ipxe/efi/efi_entropy.h
+++ /dev/null
@@ -1,35 +0,0 @@
-#ifndef _IPXE_EFI_ENTROPY_H
-#define _IPXE_EFI_ENTROPY_H
-
-/** @file
- *
- * EFI entropy source
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#include <stdint.h>
-
-#ifdef ENTROPY_EFI
-#define ENTROPY_PREFIX_efi
-#else
-#define ENTROPY_PREFIX_efi __efi_
-#endif
-
-/**
- * min-entropy per sample
- *
- * @ret min_entropy	min-entropy of each sample
- */
-static inline __always_inline min_entropy_t
-ENTROPY_INLINE ( efi, min_entropy_per_sample ) ( void ) {
-
-	/* We use essentially the same mechanism as for the BIOS
-	 * RTC-based entropy source, and so assume the same
-	 * min-entropy per sample.
-	 */
-	return MIN_ENTROPY ( 1.3 );
-}
-
-#endif /* _IPXE_EFI_ENTROPY_H */
diff --git a/src/include/ipxe/entropy.h b/src/include/ipxe/entropy.h
index d2e3ce50..108c3766 100644
--- a/src/include/ipxe/entropy.h
+++ b/src/include/ipxe/entropy.h
@@ -12,40 +12,11 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <stdint.h>
 #include <string.h>
 #include <assert.h>
-#include <ipxe/api.h>
 #include <ipxe/hash_df.h>
 #include <ipxe/sha256.h>
+#include <ipxe/tables.h>
 #include <config/entropy.h>
 
-/**
- * Calculate static inline entropy API function name
- *
- * @v _prefix		Subsystem prefix
- * @v _api_func		API function
- * @ret _subsys_func	Subsystem API function
- */
-#define ENTROPY_INLINE( _subsys, _api_func ) \
-	SINGLE_API_INLINE ( ENTROPY_PREFIX_ ## _subsys, _api_func )
-
-/**
- * Provide a entropy API implementation
- *
- * @v _prefix		Subsystem prefix
- * @v _api_func		API function
- * @v _func		Implementing function
- */
-#define PROVIDE_ENTROPY( _subsys, _api_func, _func ) \
-	PROVIDE_SINGLE_API ( ENTROPY_PREFIX_ ## _subsys, _api_func, _func )
-
-/**
- * Provide a static inline entropy API implementation
- *
- * @v _prefix		Subsystem prefix
- * @v _api_func		API function
- */
-#define PROVIDE_ENTROPY_INLINE( _subsys, _api_func ) \
-	PROVIDE_SINGLE_API_INLINE ( ENTROPY_PREFIX_ ## _subsys, _api_func )
-
 /** A noise sample */
 typedef uint8_t noise_sample_t;
 
@@ -71,56 +42,93 @@ typedef unsigned int min_entropy_t;
 #define MIN_ENTROPY( bits ) \
 	( ( min_entropy_t ) ( (bits) * MIN_ENTROPY_SCALE ) )
 
-/* Include all architecture-independent entropy API headers */
-#include <ipxe/null_entropy.h>
-#include <ipxe/efi/efi_entropy.h>
-#include <ipxe/linux/linux_entropy.h>
+/** An entropy source */
+struct entropy_source {
+	/** Name */
+	const char *name;
+	/**
+	 * min-entropy per sample
+	 *
+	 * min-entropy is defined in ANS X9.82 Part 1-2006 Section 8.3 and in
+	 * NIST SP 800-90 Appendix C.3 as
+	 *
+	 *    H_min = -log2 ( p_max )
+	 *
+	 * where p_max is the probability of the most likely sample value.
+	 *
+	 * Filled in by entropy_init().
+	 */
+	min_entropy_t min_entropy_per_sample;
+	/**
+	 * Repetition count test cutoff value
+	 *
+	 * This is the cutoff value for the Repetition Count Test
+	 * defined in ANS X9.82 Part 2 (October 2011 Draft) Section
+	 * 8.5.2.1.2.
+	 *
+	 * Filled in by entropy_init().
+	 */
+	unsigned int repetition_count_cutoff;
+	/**
+	 * Adaptive proportion test cutoff value
+	 *
+	 * This is the cutoff value for the Adaptive Proportion Test
+	 * defined in ANS X9.82 Part 2 (October 2011 Draft) Section
+	 * 8.5.2.1.3.1.2.
+	 *
+	 * Filled in by entropy_init().
+	 */
+	unsigned int adaptive_proportion_cutoff;
+	/**
+	 * Startup test count
+	 *
+	 * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.5
+	 * requires that at least one full cycle of the continuous
+	 * tests must be performed at start-up.
+	 */
+	unsigned int startup_test_count;
+	/**
+	 * Enable entropy gathering
+	 *
+	 * @ret rc		Return status code
+	 */
+	int ( * enable ) ( void );
+	/**
+	 * Disable entropy gathering
+	 *
+	 */
+	void ( * disable ) ( void );
+	/**
+	 * Get noise sample
+	 *
+	 * @ret noise		Noise sample
+	 * @ret rc		Return status code
+	 *
+	 * This is the GetNoise function defined in ANS X9.82 Part 2
+	 * (October 2011 Draft) Section 6.5.2.
+	 */
+	int ( * get_noise ) ( noise_sample_t *noise );
+};
 
-/* Include all architecture-dependent entropy API headers */
-#include <bits/entropy.h>
+/** Entropy source table */
+#define ENTROPY_SOURCES __table ( struct entropy_source, "entropy_sources" )
 
-/**
- * Enable entropy gathering
- *
- * @ret rc		Return status code
- */
-int entropy_enable ( void );
+/** Declare an entropy source */
+#define __entropy_source( order ) __table_entry ( ENTROPY_SOURCES, order )
 
-/**
- * Disable entropy gathering
+/** @defgroup entropy_source_order Entropy source order
  *
+ * @{
  */
-void entropy_disable ( void );
 
-/**
- * min-entropy per sample
- *
- * @ret min_entropy	min-entropy of each sample
- *
- * min-entropy is defined in ANS X9.82 Part 1-2006 Section 8.3 and in
- * NIST SP 800-90 Appendix C.3 as
- *
- *    H_min = -log2 ( p_max )
- *
- * where p_max is the probability of the most likely sample value.
- *
- * This must be a compile-time constant.
- */
-min_entropy_t min_entropy_per_sample ( void );
+#define ENTROPY_PREFERRED	01	/**< Preferred entropy source */
+#define ENTROPY_NORMAL		02	/**< Normal entropy source */
+#define ENTROPY_FALLBACK	03	/**< Fallback entropy source */
 
-/**
- * Get noise sample
- *
- * @ret noise		Noise sample
- * @ret rc		Return status code
- *
- * This is the GetNoise function defined in ANS X9.82 Part 2
- * (October 2011 Draft) Section 6.5.2.
- */
-int get_noise ( noise_sample_t *noise );
+/** @} */
 
-extern int get_entropy_input_tmp ( unsigned int num_samples,
-				   uint8_t *tmp, size_t tmp_len );
+extern int get_entropy_input_tmp ( min_entropy_t min_entropy, uint8_t *tmp,
+				   size_t tmp_len );
 
 /** Use SHA-256 as the underlying hash algorithm for Hash_df
  *
@@ -145,8 +153,8 @@ extern int get_entropy_input_tmp ( unsigned int num_samples,
  * each entropy source output after each GetEntropy call) as defined
  * in ANS X9.82 Part 4 (April 2011 Draft) Section 13.3.4.2.
  *
- * To minimise code size, the number of samples required is calculated
- * at compilation time.
+ * This function is inlined since the entropy amount and length inputs
+ * are always compile-time constants.
  */
 static inline __attribute__ (( always_inline )) int
 get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_len,
@@ -154,41 +162,16 @@ get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_len,
 	size_t tmp_len = ( ( ( min_entropy_bits * 2 ) + 7 ) / 8 );
 	uint8_t tmp_buf[ tmp_len ];
 	uint8_t *tmp = ( ( tmp_len > max_len ) ? tmp_buf : data );
-	double min_samples;
-	unsigned int num_samples;
 	unsigned int n;
 	int rc;
 
-	/* Sanity checks */
-	linker_assert ( ( min_entropy_per_sample() <=
-			  MIN_ENTROPY ( 8 * sizeof ( noise_sample_t ) ) ),
-			min_entropy_per_sample_is_impossibly_high );
+	/* Sanity check */
 	linker_assert ( ( min_entropy_bits <= ( 8 * max_len ) ),
 			entropy_buffer_too_small );
 
 	/* Round up minimum entropy to an integral number of bytes */
 	min_entropy_bits = ( ( min_entropy_bits + 7 ) & ~7 );
 
-	/* Calculate number of samples required to contain sufficient entropy */
-	min_samples = ( MIN_ENTROPY ( min_entropy_bits ) /
-			min_entropy_per_sample() );
-
-	/* Round up to a whole number of samples.  We don't have the
-	 * ceil() function available, so do the rounding by hand.
-	 */
-	num_samples = min_samples;
-	if ( num_samples < min_samples )
-		num_samples++;
-	linker_assert ( ( num_samples >= min_samples ), rounding_error );
-
-	/* Floating-point operations are not allowed in iPXE since we
-	 * never set up a suitable environment.  Abort the build
-	 * unless the calculated number of samples is a compile-time
-	 * constant.
-	 */
-	linker_assert ( __builtin_constant_p ( num_samples ),
-			num_samples_not_constant );
-
 	/* (Unnumbered).  The output length of the hash function shall
 	 * meet or exceed the security strength indicated by the
 	 * min_entropy parameter.
@@ -218,8 +201,10 @@ get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_len,
 	linker_assert ( __builtin_constant_p ( tmp_len ),
 			tmp_len_not_constant );
 	linker_assert ( ( n == ( 8 * tmp_len ) ), tmp_len_mismatch );
-	if ( ( rc = get_entropy_input_tmp ( num_samples, tmp, tmp_len ) ) != 0 )
+	if ( ( rc = get_entropy_input_tmp ( MIN_ENTROPY ( min_entropy_bits ),
+					    tmp, tmp_len ) ) != 0 ) {
 		return rc;
+	}
 
 	/* 6.  If ( n < min_length ), then tmp = tmp || 0^(min_length-n)
 	 * 7.  If ( n > max_length ), then tmp = df ( tmp, max_length )
@@ -242,4 +227,231 @@ get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_len,
 	}
 }
 
+/**
+ * Calculate cutoff value for the repetition count test
+ *
+ * @v min_entropy_per_sample	Min-entropy per sample
+ * @ret cutoff			Cutoff value
+ *
+ * This is the cutoff value for the Repetition Count Test defined in
+ * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.2.
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+entropy_repetition_count_cutoff ( min_entropy_t min_entropy_per_sample ) {
+	double max_repetitions;
+	unsigned int cutoff;
+
+	/* The cutoff formula for the repetition test is:
+	 *
+	 *   C = ( 1 + ( -log2(W) / H_min ) )
+	 *
+	 * where W is set at 2^(-30) (in ANS X9.82 Part 2 (October
+	 * 2011 Draft) Section 8.5.2.1.3.1).
+	 */
+	max_repetitions = ( 1 + ( MIN_ENTROPY ( 30 ) /
+				  min_entropy_per_sample ) );
+
+	/* Round up to a whole number of repetitions.  We don't have
+	 * the ceil() function available, so do the rounding by hand.
+	 */
+	cutoff = max_repetitions;
+	if ( cutoff < max_repetitions )
+		cutoff++;
+	linker_assert ( ( cutoff >= max_repetitions ), rounding_error );
+
+	/* Floating-point operations are not allowed in iPXE since we
+	 * never set up a suitable environment.  Abort the build
+	 * unless the calculated number of repetitions is a
+	 * compile-time constant.
+	 */
+	linker_assert ( __builtin_constant_p ( cutoff ),
+			repetition_count_cutoff_not_constant );
+
+	return cutoff;
+}
+
+/**
+ * Window size for the adaptive proportion test
+ *
+ * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.1.1 allows
+ * five possible window sizes: 16, 64, 256, 4096 and 65536.
+ *
+ * We expect to generate relatively few (<256) entropy samples during
+ * a typical iPXE run; the use of a large window size would mean that
+ * the test would never complete a single cycle.  We use a window size
+ * of 64, which is the smallest window size that permits values of
+ * H_min down to one bit per sample.
+ */
+#define ADAPTIVE_PROPORTION_WINDOW_SIZE 64
+
+/**
+ * Combine adaptive proportion test window size and min-entropy
+ *
+ * @v n			N (window size)
+ * @v h			H (min-entropy)
+ * @ret n_h		(N,H) combined value
+ */
+#define APC_N_H( n, h ) ( ( (n) << 8 ) | (h) )
+
+/**
+ * Define a row of the adaptive proportion cutoff table
+ *
+ * @v h			H (min-entropy)
+ * @v c16		Cutoff for N=16
+ * @v c64		Cutoff for N=64
+ * @v c256		Cutoff for N=256
+ * @v c4096		Cutoff for N=4096
+ * @v c65536		Cutoff for N=65536
+ */
+#define APC_TABLE_ROW( h, c16, c64, c256, c4096, c65536)	   \
+	case APC_N_H ( 16, h ) :	return c16;		   \
+	case APC_N_H ( 64, h ) :	return c64;   		   \
+	case APC_N_H ( 256, h ) :	return c256;		   \
+	case APC_N_H ( 4096, h ) :	return c4096;		   \
+	case APC_N_H ( 65536, h ) :	return c65536;
+
+/** Value used to represent "N/A" in adaptive proportion cutoff table */
+#define APC_NA 0
+
+/**
+ * Look up value in adaptive proportion test cutoff table
+ *
+ * @v n			N (window size)
+ * @v h			H (min-entropy)
+ * @ret cutoff		Cutoff
+ *
+ * This is the table of cutoff values defined in ANS X9.82 Part 2
+ * (October 2011 Draft) Section 8.5.2.1.3.1.2.
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+entropy_adaptive_proportion_cutoff_lookup ( unsigned int n, unsigned int h ) {
+	switch ( APC_N_H ( n, h ) ) {
+		APC_TABLE_ROW (  1, APC_NA,     51,    168,   2240,  33537 );
+		APC_TABLE_ROW (  2, APC_NA,     35,    100,   1193,  17053 );
+		APC_TABLE_ROW (  3,     10,     24,     61,    643,   8705 );
+		APC_TABLE_ROW (  4,      8,     16,     38,    354,   4473 );
+		APC_TABLE_ROW (  5,      6,     12,     25,    200,   2321 );
+		APC_TABLE_ROW (  6,      5,      9,     17,    117,   1220 );
+		APC_TABLE_ROW (  7,      4,      7,     15,     71,    653 );
+		APC_TABLE_ROW (  8,      4,      5,      9,     45,    358 );
+		APC_TABLE_ROW (  9,      3,      4,      7,     30,    202 );
+		APC_TABLE_ROW ( 10,      3,      4,      5,     21,    118 );
+		APC_TABLE_ROW ( 11,      2,      3,      4,     15,     71 );
+		APC_TABLE_ROW ( 12,      2,      3,      4,     11,     45 );
+		APC_TABLE_ROW ( 13,      2,      2,      3,      9,     30 );
+		APC_TABLE_ROW ( 14,      2,      2,      3,      7,     21 );
+		APC_TABLE_ROW ( 15,      1,      2,      2,      6,     15 );
+		APC_TABLE_ROW ( 16,      1,      2,      2,      5,     11 );
+		APC_TABLE_ROW ( 17,      1,      1,      2,      4,      9 );
+		APC_TABLE_ROW ( 18,      1,      1,      2,      4,      7 );
+		APC_TABLE_ROW ( 19,      1,      1,      1,      3,      6 );
+		APC_TABLE_ROW ( 20,      1,      1,      1,      3,      5 );
+	default:
+		return APC_NA;
+	}
+}
+
+/**
+ * Calculate cutoff value for the adaptive proportion test
+ *
+ * @v min_entropy_per_sample	Min-entropy per sample
+ * @ret cutoff			Cutoff value
+ *
+ * This is the cutoff value for the Adaptive Proportion Test defined
+ * in ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.1.2.
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+entropy_adaptive_proportion_cutoff ( min_entropy_t min_entropy_per_sample ) {
+	unsigned int h;
+	unsigned int n;
+	unsigned int cutoff;
+
+	/* Look up cutoff value in cutoff table */
+	n = ADAPTIVE_PROPORTION_WINDOW_SIZE;
+	h = ( min_entropy_per_sample / MIN_ENTROPY_SCALE );
+	cutoff = entropy_adaptive_proportion_cutoff_lookup ( n, h );
+
+	/* Fail unless cutoff value is a compile-time constant */
+	linker_assert ( __builtin_constant_p ( cutoff ),
+			adaptive_proportion_cutoff_not_constant );
+
+	/* Fail if cutoff value is N/A */
+	linker_assert ( ( cutoff != APC_NA ),
+			adaptive_proportion_cutoff_not_applicable );
+
+	return cutoff;
+}
+
+/**
+ * Calculate number of samples required for startup tests
+ *
+ * @v repetition_count_cutoff	Repetition count test cutoff value
+ * @v adaptive_proportion_cutoff Adaptive proportion test cutoff value
+ * @ret num_samples		Number of samples required
+ *
+ * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.5 requires
+ * that at least one full cycle of the continuous tests must be
+ * performed at start-up.
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+entropy_startup_test_count ( unsigned int repetition_count_cutoff,
+			     unsigned int adaptive_proportion_cutoff ) {
+	unsigned int num_samples;
+
+	/* At least max(N,C) samples shall be generated by the noise
+	 * source for start-up testing.
+	 */
+	num_samples = repetition_count_cutoff;
+	if ( num_samples < adaptive_proportion_cutoff )
+		num_samples = adaptive_proportion_cutoff;
+	linker_assert ( __builtin_constant_p ( num_samples ),
+			startup_test_count_not_constant );
+
+	return num_samples;
+}
+
+/**
+ * Initialise entropy source
+ *
+ * @v source			Entropy source
+ * @v min_entropy_per_sample	Min-entropy per sample
+ *
+ * The cutoff value calculations for the repetition count test and the
+ * adaptive proportion test are provided as static inline functions
+ * since the results will always be compile-time constants.
+ */
+static inline __attribute__ (( always_inline )) void
+entropy_init ( struct entropy_source *source,
+	       min_entropy_t min_entropy_per_sample ) {
+	unsigned int repetition_count_cutoff;
+	unsigned int adaptive_proportion_cutoff;
+	unsigned int startup_test_count;
+
+	/* Sanity check */
+	linker_assert ( min_entropy_per_sample > MIN_ENTROPY ( 0 ),
+			min_entropy_per_sample_is_zero );
+	linker_assert ( ( min_entropy_per_sample <=
+			  MIN_ENTROPY ( 8 * sizeof ( noise_sample_t ) ) ),
+			min_entropy_per_sample_is_impossibly_high );
+
+	/* Calculate test cutoff values */
+	repetition_count_cutoff =
+		entropy_repetition_count_cutoff ( min_entropy_per_sample );
+	adaptive_proportion_cutoff =
+		entropy_adaptive_proportion_cutoff ( min_entropy_per_sample );
+	startup_test_count =
+		entropy_startup_test_count ( repetition_count_cutoff,
+					     adaptive_proportion_cutoff );
+
+	/* Record min-entropy per sample and test cutoff values */
+	source->min_entropy_per_sample = min_entropy_per_sample;
+	source->repetition_count_cutoff = repetition_count_cutoff;
+	source->adaptive_proportion_cutoff = adaptive_proportion_cutoff;
+	source->startup_test_count = startup_test_count;
+}
+
+extern int entropy_enable ( void );
+extern void entropy_disable ( void );
+extern int get_noise ( noise_sample_t *noise );
+
 #endif /* _IPXE_ENTROPY_H */
diff --git a/src/include/ipxe/linux/linux_entropy.h b/src/include/ipxe/linux/linux_entropy.h
deleted file mode 100644
index ea8c1f16..00000000
--- a/src/include/ipxe/linux/linux_entropy.h
+++ /dev/null
@@ -1,34 +0,0 @@
-#ifndef _IPXE_LINUX_ENTROPY_H
-#define _IPXE_LINUX_ENTROPY_H
-
-/** @file
- *
- * /dev/random-based entropy source
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#ifdef ENTROPY_LINUX
-#define ENTROPY_PREFIX_linux
-#else
-#define ENTROPY_PREFIX_linux __linux_
-#endif
-
-/**
- * min-entropy per sample
- *
- * @ret min_entropy	min-entropy of each sample
- */
-static inline __always_inline min_entropy_t
-ENTROPY_INLINE ( linux, min_entropy_per_sample ) ( void ) {
-
-	/* linux_get_noise() reads a single byte from /dev/random,
-	 * which is supposed to block until a sufficient amount of
-	 * entropy is available.  We therefore assume that each sample
-	 * contains exactly 8 bits of entropy.
-	 */
-	return MIN_ENTROPY ( 8.0 );
-}
-
-#endif /* _IPXE_LINUX_ENTROPY_H */
diff --git a/src/include/ipxe/null_entropy.h b/src/include/ipxe/null_entropy.h
deleted file mode 100644
index 5a6bb621..00000000
--- a/src/include/ipxe/null_entropy.h
+++ /dev/null
@@ -1,52 +0,0 @@
-#ifndef _IPXE_NULL_ENTROPY_H
-#define _IPXE_NULL_ENTROPY_H
-
-/** @file
- *
- * Nonexistent entropy source
- *
- * This source provides no entropy and must NOT be used in a
- * security-sensitive environment.
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#include <stdint.h>
-
-#ifdef ENTROPY_NULL
-#define ENTROPY_PREFIX_null
-#else
-#define ENTROPY_PREFIX_null __null_
-#endif
-
-static inline __always_inline int
-ENTROPY_INLINE ( null, entropy_enable ) ( void ) {
-	/* Do nothing */
-	return 0;
-}
-
-static inline __always_inline void
-ENTROPY_INLINE ( null, entropy_disable ) ( void ) {
-	/* Do nothing */
-}
-
-static inline __always_inline min_entropy_t
-ENTROPY_INLINE ( null, min_entropy_per_sample ) ( void ) {
-	/* Actual amount of min-entropy is zero.  To avoid
-	 * division-by-zero errors and to allow compilation of
-	 * entropy-consuming code, pretend to have 1 bit of entropy in
-	 * each sample.
-	 */
-	return MIN_ENTROPY ( 1.0 );
-}
-
-static inline __always_inline int
-ENTROPY_INLINE ( null, get_noise ) ( noise_sample_t *noise ) {
-
-	/* All sample values are constant */
-	*noise = 0x01;
-
-	return 0;
-}
-
-#endif /* _IPXE_NULL_ENTROPY_H */