[PATCH v3 0/6] 32bit ARM branch predictor hardening

[Florian Fainelli]

On 01/31/2018 04:45 AM, Hanjun Guo wrote:
> On 2018/1/29 22:58, Nishanth Menon wrote:
>> On Mon, Jan 29, 2018 at 5:36 AM, Hanjun Guo <guohanjun at huawei.com> wrote:
>> [...]
>>
>>> By the way, this patch set just enable branch predictor hardening
>>> on arm32 unconditionally, but some of machines (such as wireless
>>> network base station) will not be exposed to user to take advantage
>>> of variant 2, and those machines will be pretty sensitive for
>>> performance, so can we introduce Kconfig or boot option to disable
>>> branch predictor hardening as an option?
>>
>> I am curious: Have you seen performance degradation with this series?
>> If yes, is it possible to share the information?
> 
> Sorry for the late reply, the performance data for context switch (CFS)
> is about 6%~12% drop (A9 based machine) for the first around test, but
> the data is not stable, I need to retest then I will update here.

What tool did you use to measure this? On a Brahma-B15 platform clocked
at 1.5Ghz, across kernels 4.1, 4.9 (4.15 in progress as we speak), I
measured the following, with two memory configurations, one giving 256MB
of usable memory, another giving 3GB of usable memory, results below are
only the most extreme 256MB case. This is running 13 groups because the
ASID space is 256bits so this should force at least two full ASID
generation rollovers (assuming the logic is correct here).

for i in $(seq 0 9)
do
	hackbench 13 process 10000
done

Average values, in seconds:

1) 4.1.45, ACTLR[0] = 0, no spectre variant 2 patches: 114,2666
2) 4.1.45, ACTLR[0] = 1, no spectre variant 2 patches: 114,2952
3) 4.1.45, ACTLR[0] =1 , spectre variant 2 patches: 115,5853

=> 3) is a 1.15% degradation against 1)

4.9.51, ACTLR[0] = 0, no spectre variant 2 patches: 130,7676
4.9.51, ACTLR[0] = 1, no spectre variant 2 patches: 130,6848
4.9.51, ACTLR[0] =1 , spectre variant 2 patches: 132,4274

=> 3) is a 1.26% degradation against 1)

The relative differences between 4.1 and 4.9 appear consistent (with 4.9
being slower for a reason I ignore).

Marc, are there any performance tests/results that you ran that you
could share?
-- 
Florian