x86_64: generate a seed for the RNG with RDRAND when available

arch/x86_64/include/kernel/machine/random.h

@@ -0,0 +1,10 @@
+#ifndef KERNEL_X86_64_RANDOM_H_
+#define KERNEL_X86_64_RANDOM_H_
+
+#include <stdbool.h>
+#include <stdint.h>
+
+extern bool ml_hwrng_available(void);
+extern uint64_t ml_hwrng_generate(void);
+
+#endif

arch/x86_64/init.c

@@ -10,12 +10,14 @@
 #include <kernel/init.h>
 #include <kernel/libc/stdio.h>
 #include <kernel/machine/cpu.h>
+#include <kernel/machine/random.h>
 #include <kernel/memblock.h>
 #include <kernel/object.h>
 #include <kernel/percpu.h>
 #include <kernel/pmap.h>
 #include <kernel/printk.h>
 #include <kernel/types.h>
+#include <kernel/util.h>
 #include <kernel/vm.h>
 
 #define PTR32(x)            ((void *)((uintptr_t)(x)))
@@ -123,6 +125,18 @@ int ml_init(uintptr_t arg)
 		reserve_end = bsp.mod_base + bsp.mod_size;
 	}
 
+	if (ml_hwrng_available()) {
+		printk("cpu: ardware RNG available");
+		uint64_t seed = ml_hwrng_generate();
+		printk("cpu: RNG seed=%zx", seed);
+		init_random(seed);
+	} else {
+		printk("cpu: hardware RNG unavailable");
+		uint64_t seed = 0xeddc4c8a679dc23f;
+		printk("cpu: RNG seed=%zx", seed);
+		init_random(seed);
+	}
+
 	early_vm_init(reserve_end);
 
 	e820_scan(PTR32(mb->mmap_addr), mb->mmap_length);

arch/x86_64/random.S

@@ -0,0 +1,43 @@
+.code64
+
+.global ml_hwrng_available
+.type ml_hwrng_available, @function
+
+ml_hwrng_available:
+	push %rbp
+	mov %rsp, %rbp
+	push %rbx
+	push %rdx
+
+	mov $1, %eax
+	mov $0, %ecx
+
+	cpuid
+
+	shr $30, %ecx
+	and $1, %ecx
+	mov %ecx, %eax
+
+	pop %rdx
+	pop %rbx
+	pop %rbp
+	ret
+
+.global ml_hwrng_generate
+.type ml_hwrng_generate, @function
+
+ml_hwrng_generate:
+	push %rbp
+	mov %rsp, %rbp
+
+	mov $100, %rcx
+.retry:
+	rdrand %rax
+	jc .done
+	loop .retry
+
+.fail:
+	mov $0, %rax
+.done:
+	pop %rbp
+	ret