Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

Borislav Petkov
2015-04-28T13:40:37Z

On Mon, Apr 27, 2015 at 09:45:12PM +0200, Borislav Petkov wrote:
> > Maybe you are measuring random noise.
>
> Yeah. Last exercise tomorrow. Let's see what those numbers would look
> like.

Right, so with Mel's help, I did a simple microbenchmark to measure how
many cycles a syscall (getpid()) needs on 4.1-rc1 and with your patch.

4.1-rc1
-------
Running 60 times, 10000000 loops per run.
Cycles: 3977233027, cycles/syscall: 397.723303
Cycles: 3964979519, cycles/syscall: 396.497952
Cycles: 3962470261, cycles/syscall: 396.247026
Cycles: 3963524693, cycles/syscall: 396.352469
Cycles: 3962853704, cycles/syscall: 396.285370
Cycles: 3964603727, cycles/syscall: 396.460373
Cycles: 3964758926, cycles/syscall: 396.475893
Cycles: 3965268019, cycles/syscall: 396.526802
Cycles: 3962198683, cycles/syscall: 396.219868
...

4.1-rc1 + 17be0aec74fb036eb4eb32c2268f3420a034762b from tip
-----------------------------------------------------------
Running 60 times, 10000000 loops per run.
Cycles: 3973575441, cycles/syscall: 397.357544
Cycles: 3963999393, cycles/syscall: 396.399939
Cycles: 3962613575, cycles/syscall: 396.261357
Cycles: 3963440408, cycles/syscall: 396.344041
Cycles: 3963475255, cycles/syscall: 396.347526
Cycles: 3964471785, cycles/syscall: 396.447179
Cycles: 3962890513, cycles/syscall: 396.289051
Cycles: 3964940114, cycles/syscall: 396.494011
Cycles: 3964186426, cycles/syscall: 396.418643
...

So yeah, your patch is fine - provided I've done everything right. Here's
the microbenchmark:

---
/*
* How to run:
*
* taskset -c <cpunum> ./sys
*/
#include <stdio.h>
#include <sys/syscall.h>
#include <stdlib.h>
#include <unistd.h>

typedef unsigned long long u64;

#define DECLARE_ARGS(val, low, high) unsigned low, high
#define EAX_EDX_VAL(val, low, high) ((low) | ((u64)(high) << 32))
#define EAX_EDX_ARGS(val, low, high) "a" (low), "d" (high)
#define EAX_EDX_RET(val, low, high) "=a" (low), "=d" (high)

static __always_inline unsigned long long rdtsc(void)
{
DECLARE_ARGS(val, low, high);

asm volatile("rdtsc" : EAX_EDX_RET(val, low, high));

return EAX_EDX_VAL(val, low, high);
}

static long my_getpid(void)
{
long ret;
asm volatile ("syscall" :
"=a" (ret) :
"a" (SYS_getpid) :
"memory", "cc", "rcx", "r11");
return ret;
}

static inline u64 read_tsc(void)
{
u64 ret;

asm volatile("mfence");
ret = rdtsc();
asm volatile("mfence");

return ret;
}

int main()
{
int i, j;
u64 p1, p2;
u64 count = 0;

#define TIMES 60
#define LOOPS 10000000ULL

printf("Running %d times, %lld loops per run.\n", TIMES, LOOPS);

for (j = 0; j < TIMES; j++) {
for (i = 0; i < LOOPS; i++) {
p1 = read_tsc();
my_getpid();
p2 = read_tsc();

count += (p2 - p1);
}

printf("Cycles: %lld, cycles/syscall: %f\n",
count, (double)count / LOOPS);

count = 0;
}

return 0;
}

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
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