netdev - Re: [PATCH v2 net-next 00/12] net: stmmac: Clean-up and tune-up

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Message-ID: <64edab6c-0bbf-fd8a-6be6-aaf802f6d12f@gmail.com>
Date:   Wed, 16 May 2018 12:01:26 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     David Miller <davem@...emloft.net>, Jose.Abreu@...opsys.com
Cc:     netdev@...r.kernel.org, Joao.Pinto@...opsys.com,
        Vitor.Soares@...opsys.com, peppe.cavallaro@...com,
        alexandre.torgue@...com
Subject: Re: [PATCH v2 net-next 00/12] net: stmmac: Clean-up and tune-up

On 05/16/2018 11:56 AM, David Miller wrote:
> From: Jose Abreu <Jose.Abreu@...opsys.com>
> Date: Wed, 16 May 2018 13:50:42 +0100
> 
>> David raised some rightfull constrains about the use of indirect callbacks in
>> the code. I did iperf tests with and without patches 3-12 and the performance
>> remained equal. I guess for 1Gb/s and because my setup has a powerfull
>> processor these patches don't affect the performance.
> 
> Does your cpu need Spectre v1 and v2 workarounds which cause indirect calls to
> be extremely expensive?

Given how widespread stmmac is within the ARM CPU's ecosystem, the
answer is more than likely yes.

To get a better feeling of whether your indirect branches introduce a
difference, either don't run the CPU at full speed (e.g: use cpufreq to
slow it down), and/or profile the number of cycles and instruction cache
hits/miss ratio for the functions called in hot-path.
-- 
Florian