nops in virtual pc x86

From: David Sanders
Date: Fri Sep 12 2008 - 17:12:13 EST

Well, I thought we determined that multibyte nops were causing virtual pc to
die and removing them made it work fine. Then why do I see this
$ cat /proc/cpuinfo | grep nopl

flags : fpu vme pse tsc msr pae cx8 sep pge cmov acpi mmx fxsr sse
sse2 constant_tsc up nopl lahf_lm

It seems the detection code in common.c is saying nops are supported. Huh?

SO I ran this code:
#include <stdio.h>
#include <time.h>

int main()
unsigned char nopl1[2] = { 0x90, 0xc3 };
unsigned char nopl2[3] = { 0x66, 0x90, 0xc3 };
unsigned char nopl3[4] = { 0x0f, 0x1f, 0x00, 0xc3 };
unsigned char nopl4[5] = { 0x0f, 0x1f, 0x40, 0x00, 0xc3 };
unsigned char nopl5[6] = { 0x0f, 0x1f, 0x44, 0x00, 0x00, 0xc3 };
unsigned char nopl6[7] = { 0x66, 0x0f, 0x1f, 0x44, 0x00, 0x00, 0xc3 };
unsigned char nopl7[8] = { 0x0f, 0x1f, 0x80, 0x00, 0x00, 0x00, 0x00, 0xc3 };
unsigned char nopl8[9] = { 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00,
0xc3 };

int i;
for (i=0;i<100000;i++) {
((void (*)()) nopl1)();
((void (*)()) nopl2)();
((void (*)()) nopl3)();
((void (*)()) nopl4)();
((void (*)()) nopl5)();
((void (*)()) nopl6)();
((void (*)()) nopl7)();
((void (*)()) nopl8)();

printf("Executed 800,000 NOPLs in %d ticks with no errors.\n", clock());
return 0;

It seems I can run 800000 nopl's without anyone complaining in user space.

Could it perhaps depend on the context the nops appear in?

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