linux-kernel - Re: [PATCH v9 15/17] x86/split_lock: Add documentation for split lock detection interface

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Date:   Wed, 26 Jun 2019 23:51:59 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v9 15/17] x86/split_lock: Add documentation for split
 lock detection interface

On Tue, 18 Jun 2019, Fenghua Yu wrote:

> It is useful for development and debugging to document the new debugfs
> interface /sys/kernel/debug/x86/split_lock_detect.
> 
> A new debugfs documentation is created to describe the split lock detection
> interface. In the future, more entries may be added in the documentation to
> describe other interfaces under /sys/kernel/debug/x86 directory.
> 
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> ---
>  Documentation/ABI/testing/debugfs-x86 | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>  create mode 100644 Documentation/ABI/testing/debugfs-x86
> 
> diff --git a/Documentation/ABI/testing/debugfs-x86 b/Documentation/ABI/testing/debugfs-x86
> new file mode 100644
> index 000000000000..17a1e9ed6712
> --- /dev/null
> +++ b/Documentation/ABI/testing/debugfs-x86
> @@ -0,0 +1,21 @@
> +What:		/sys/kernel/debugfs/x86/split_lock_detect
> +Date:		May 2019
> +Contact:	Linux kernel mailing list <linux-kernel@...r.kernel.org>
> +Description:	(RW) Control split lock detection on Intel Tremont and
> +		future CPUs
> +
> +		Reads return split lock detection status:
> +			0: disabled
> +			1: enabled
> +
> +		Writes enable or disable split lock detection:
> +			The first character is one of 'Nn0' or [oO][fF] for off
> +			disables the feature.
> +			The first character is one of 'Yy1' or [oO][nN] for on
> +			enables the feature.
> +
> +		Please note the interface only shows or controls global setting.
> +		During run time, split lock detection on one CPU may be
> +		disabled if split lock operation in kernel code happens on
> +		the CPU. The interface doesn't show or control split lock
> +		detection on individual CPU.

But it should show that the debug output for the kernel has been globally
disabled instead of merily stating 'enabled' while it's already disabled on
a bunch on CPUs. I fundamentally despise inconsistent information. Even in
debugfs files inconsistency is a pain because it make debugging via mail/
bugzille etc. unnecessarily cumbersome.

Thanks,

	tglx