linux-kernel - RE: [Patch v11 4/5] x86/cpufeature: enable RING3MWAIT for Knights Landing

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Date:   Fri, 23 Dec 2016 18:19:06 +0000
From:   "Andrejczuk, Grzegorz" <grzegorz.andrejczuk@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Luc, Piotr" <Piotr.Luc@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: RE: [Patch v11 4/5] x86/cpufeature: enable RING3MWAIT for Knights
 Landing

On Thursday, December 22, 2016 3:21 PM Thomas Gleixner wrote:
> Changing types to match is the proper solution to all problems? You cannot
> just change types to make the compiler happy. You have to check what type
> is expected for it in the places which consume it, including compat mode.

HWCAP is u32 on x86 architecture, HWCAP2 on other architectures is uint.
I think I will be consistent with architecture and use u32 and bit OR operator.
It should be handled same way as ELF_HWCAP in user space and in 32 bits.
Please let me know if you agree to this solution. 

> What has the MSR to do with ELF_HWCAP2? ELF_HWCAP2 is a system global
> variable. The MSR is of course per hardware thread.

You are right - nothing. I can set HWCAP2 once for boot cpu.
To achieve that I need to distinguish boot cpu in probe_xeon_phi_r3mwait.
>From looking around the kernel code I suspect it can be done like below:

if (c == &boot_cpu_data)
        ELF_HWCAP2 |= HWCAP2_RING3MWAIT;

If there's a better way, please let me know.  

> CPU bringup is serialized, i.e. init_intel() cannot run concurrently on
> different CPUs.

Thank you for your explanation, time and effort.
Merry Christmas,
Grzegorz