Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64 with MCP51 laptops

Rene Herman
2007-12-11T15:31:12Z

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On 11-12-07 15:15, Rene Herman wrote:

> On 11-12-07 14:32, Paul Rolland wrote:
>
>>> On 11-12-07 13:08, David Newall wrote:
>>>
>>>> Rene Herman wrote:
>>
>>> (*) some local testing shows it to be almost exactly that for both
>>> out and in on my own PC -- a little over. If anyone cares, see
>>> attached little test program. The "little over" I don't worry about.
>>> 0 us delay is also fine for me and if any code was _that_ fragile it
>>> would have broken long ago.
>>
>> Some results :
>
> Okay, these vary to wildly for you and might I suppose be a serialising
> artifact or some such. Give me a bit and I'll try to improve it...

This might be a bit more constant, I suppose. This serialises with cpuid.
Don't see a difference locally, but perhaps you do.

On a Duron 1300 with an actual ISA bus, "out" is between 1300 and 1600 for
me and "in" between 1200 and 1500 with a few flukes above that which will I
suppose be caused by the bus (ISA _or_ PCI) being momentarily busy or some
such...

Rene.

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#include <stdlib.h>
#include <stdio.h>

#include <sys/io.h>

#define LOOPS 1000

unsigned long cycles[LOOPS];

int main(void)
{
unsigned long overhead;
unsigned long total;
int i;

if (iopl(3) < 0) {
perror("iopl");
return EXIT_FAILURE;
}

/* pull it in */
for (i = 0; i < LOOPS; i++)
cycles[i] = 0;

asm volatile ("cli");
for (i = 0; i < LOOPS; i++)
asm (
"xor %%eax, %%eax \n\t"
"cpuid \n\t"
"rdtsc \n\t"
"movl %%eax, %%esi \n\t"
"xor %%eax, %%eax \n\t"
"cpuid \n\t"
"rdtsc \n\t"
"subl %%esi, %%eax \n\t"

: "=a" (cycles[i]) : : "ecx", "edx", "ebx", "esi");
asm volatile ("sti");

overhead = 0;
for (i = 0; i < LOOPS; i++)
overhead += cycles[i];

asm volatile ("cli");
for (i = 0; i < LOOPS; i++)
asm (
"xor %%eax, %%eax \n\t"
"cpuid \n\t"
"rdtsc \n\t"
"movl %%eax, %%esi \n\t"
"outb %%al, $0x80 \n\t"
"xor %%eax, %%eax \n\t"
"cpuid \n\t"
"rdtsc \n\t"
"subl %%esi, %%eax \n\t"

: "=a" (cycles[i]) : : "ecx", "edx", "ebx", "esi");
asm volatile ("sti");

total = 0;
for (i = 0; i < LOOPS; i++)
total += cycles[i];
total -= overhead;

printf("out: %lu\n", total / LOOPS);

asm volatile ("cli");
for (i = 0; i < LOOPS; i++)
asm (
"xor %%eax, %%eax \n\t"
"cpuid \n\t"
"rdtsc \n\t"
"movl %%eax, %%esi \n\t"
"inb $0x80, %%al \n\t"
"xor %%eax, %%eax \n\t"
"cpuid \n\t"
"rdtsc \n\t"
"subl %%esi, %%eax \n\t"

: "=a" (cycles[i]) : : "ecx", "edx", "ebx", "esi");
asm volatile ("sti");

total = 0;
for (i = 0; i < LOOPS; i++)
total += cycles[i];
total -= overhead;

printf("in : %lu\n", total / LOOPS);

return EXIT_SUCCESS;
}