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

Borislav Petkov
2015-04-27T14:39:24Z

On Sun, Apr 26, 2015 at 04:39:38PM -0700, Andy Lutomirski wrote:
> I know it would be ugly, but would it be worth saving two bytes by
> using ALTERNATIVE "jmp 1f", "shl ...", ...?

Damn, it is actually visible even that saving the unconditional forward
JMP makes the numbers marginally nicer (E: row). So I guess we'll be
dropping the forward JMP too.

A: 2835570.145246 cpu-clock (msec) ( +- 0.02% ) [100.00%]
B: 2833364.074970 cpu-clock (msec) ( +- 0.04% ) [100.00%]
C: 2834708.335431 cpu-clock (msec) ( +- 0.02% ) [100.00%]
D: 2835055.118431 cpu-clock (msec) ( +- 0.01% ) [100.00%]
E: 2833115.118624 cpu-clock (msec) ( +- 0.06% ) [100.00%]

A: 2835570.099981 task-clock (msec) # 3.996 CPUs utilized ( +- 0.02% ) [100.00%]
B: 2833364.073633 task-clock (msec) # 3.996 CPUs utilized ( +- 0.04% ) [100.00%]
C: 2834708.350387 task-clock (msec) # 3.996 CPUs utilized ( +- 0.02% ) [100.00%]
D: 2835055.094383 task-clock (msec) # 3.996 CPUs utilized ( +- 0.01% ) [100.00%]
E: 2833115.145292 task-clock (msec) # 3.996 CPUs utilized ( +- 0.06% ) [100.00%]

A: 5,591,213,166,613 cycles # 1.972 GHz ( +- 0.03% ) [75.00%]
B: 5,585,023,802,888 cycles # 1.971 GHz ( +- 0.03% ) [75.00%]
C: 5,587,983,212,758 cycles # 1.971 GHz ( +- 0.02% ) [75.00%]
D: 5,584,838,532,936 cycles # 1.970 GHz ( +- 0.03% ) [75.00%]
E: 5,583,979,727,842 cycles # 1.971 GHz ( +- 0.05% ) [75.00%]

cycles is the lowest, nice.

A: 3,106,707,101,530 instructions # 0.56 insns per cycle ( +- 0.01% ) [75.00%]
B: 3,106,632,251,528 instructions # 0.56 insns per cycle ( +- 0.00% ) [75.00%]
C: 3,106,265,958,142 instructions # 0.56 insns per cycle ( +- 0.00% ) [75.00%]
D: 3,106,294,801,185 instructions # 0.56 insns per cycle ( +- 0.00% ) [75.00%]
E: 3,106,381,223,355 instructions # 0.56 insns per cycle ( +- 0.01% ) [75.00%]

Understandable - we end up executing 5 insns more:

ffffffff815b90ac: 66 66 66 90 data16 data16 xchg %ax,%ax
ffffffff815b90b0: 66 66 66 90 data16 data16 xchg %ax,%ax
ffffffff815b90b4: 66 66 66 90 data16 data16 xchg %ax,%ax
ffffffff815b90b8: 66 66 66 90 data16 data16 xchg %ax,%ax
ffffffff815b90bc: 90 nop

A: 683,676,044,429 branches # 241.107 M/sec ( +- 0.01% ) [75.00%]
B: 683,670,899,595 branches # 241.293 M/sec ( +- 0.01% ) [75.00%]
C: 683,675,772,858 branches # 241.180 M/sec ( +- 0.01% ) [75.00%]
D: 683,683,533,664 branches # 241.154 M/sec ( +- 0.00% ) [75.00%]
E: 683,648,518,667 branches # 241.306 M/sec ( +- 0.01% ) [75.00%]

Lowest.

A: 43,829,535,008 branch-misses # 6.41% of all branches ( +- 0.02% ) [75.00%]
B: 43,844,118,416 branch-misses # 6.41% of all branches ( +- 0.03% ) [75.00%]
C: 43,819,871,086 branch-misses # 6.41% of all branches ( +- 0.02% ) [75.00%]
D: 43,795,107,998 branch-misses # 6.41% of all branches ( +- 0.02% ) [75.00%]
E: 43,801,985,070 branch-misses # 6.41% of all branches ( +- 0.02% ) [75.00%]

That looks like noise to me - we shouldn't be getting more branch misses
with the E: version.

A: 2,030,357 context-switches # 0.716 K/sec ( +- 0.06% ) [100.00%]
B: 2,029,313 context-switches # 0.716 K/sec ( +- 0.05% ) [100.00%]
C: 2,028,566 context-switches # 0.716 K/sec ( +- 0.06% ) [100.00%]
D: 2,028,895 context-switches # 0.716 K/sec ( +- 0.06% ) [100.00%]
E: 2,031,008 context-switches # 0.717 K/sec ( +- 0.09% ) [100.00%]

A: 52,421 migrations # 0.018 K/sec ( +- 1.13% )
B: 52,049 migrations # 0.018 K/sec ( +- 1.02% )
C: 51,365 migrations # 0.018 K/sec ( +- 0.92% )
D: 51,766 migrations # 0.018 K/sec ( +- 1.11% )
E: 53,047 migrations # 0.019 K/sec ( +- 1.08% )

A: 709.528485252 seconds time elapsed ( +- 0.02% )
B: 708.976557288 seconds time elapsed ( +- 0.04% )
C: 709.312844791 seconds time elapsed ( +- 0.02% )
D: 709.400050112 seconds time elapsed ( +- 0.01% )
E: 708.914562508 seconds time elapsed ( +- 0.06% )

Nice.

--
Regards/Gruss,
Boris.

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