linux-kernel - Re: [PATCH] x86: Optimize load_mm_cr4 to load_mm_cr4

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Date:   Tue, 18 Jun 2019 16:56:03 +0200
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Optimize load_mm_cr4 to load_mm_cr4_irqsoff

On 18.06.19 16:41, Dave Hansen wrote:
> On 6/18/19 12:32 AM, Jan Kiszka wrote:
>> Thus, we can avoid disabling interrupts again in cr4_set/clear_bits.
> 
> Seems reasonable.
> 
> Your *_irqsoff() variants need lockdep_assert_irqs_disabled(), at least,
> though.

It inherits this check via __cr4_set, so I didn't add that to the outer path.

> 
> Can you talk a bit about the motivation here, though?  Did you encounter
> some performance issue that led you to make this patch, or was it simply
> an improvement you realized you could make from code inspection?
> 

I ran into a downstream issue adjusting patches to this code. In a nutshell, 
Xenomai has something like an NMI context over most Linux code that shares some 
codepaths with the kernel, though. One of them is switch_mm_irqs_off, and there 
our checking logic warned. The rest was code inspection.

For upstream, this is a micro-optimization. But given that something like 
switch_mm is in the path, I thought it's worth to propose.

Jan

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