[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>

1 files changed, 0 insertions, 35 deletions

diff --git a/src/include/ipxe/efi/efi_entropy.h b/src/include/ipxe/efi/efi_entropy.h
deleted file mode 100644
index 5b16fd7..0000000
--- 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 */