holbe@ideapad ~ % cat via-rng-test.c
#include <stdio.h>
#include <string.h>

static inline unsigned long xstore(unsigned long *addr, unsigned long edx)
{
	unsigned long eax_out;
	unsigned long ecx = 0xaa55aa55;
	int ts_state;

	printf("ecx: %08lx\tedx: %08lx\tedi: %p\n", ecx, edx, addr);

	asm(".byte 0x0F,0xA7,0xC0 /* xstore %%edi (addr=%0) */"
		: "=m" (*addr), "=a" (eax_out), "+D" (addr), "+d" (edx), "+c" (ecx));

	printf("ecx: %08lx\tedx: %08lx\tedi: %p\n", ecx, edx, addr);

	return eax_out;
}

int main(void) {
	unsigned long addr[8];

	memset(addr, 0, sizeof(addr));

	printf("%p: %08lx\n", addr, *(unsigned long *)addr);
	xstore(addr, 0xffffff03); /* 8 rand bits, 32 stored bits */
	printf("%p: %08lx\n", addr, *(unsigned long *)addr);

	return 0;
}
holbe@ideapad ~ % gcc -o via-rng-test via-rng-test.c
holbe@ideapad ~ % ./via-rng-test
0xbff09d40: 00000000
ecx: aa55aa55   edx: ffffff03   edi: 0xbff09d40
ecx: aa55aa55   edx: 00000003   edi: 0xbff09d48
0xbff09d40: 339d4525
holbe@ideapad ~ %