linux-kernel - nops in virtual pc x86

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Date:	Fri, 12 Sep 2008 17:11:47 -0400
From:	David Sanders <linux@...dersweb.net>
To:	Linux Kernel <linux-kernel@...r.kernel.org>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: nops in virtual pc x86

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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