linux-kernel - Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel

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Message-ID: <20200125203312.GE4369@zn.tnic>
Date:   Sat, 25 Jan 2020 21:33:12 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel

On Sat, Jan 25, 2020 at 09:12:21PM +0100, Peter Zijlstra wrote:
> > Blame PeterZ for that. For now I'd like to add the duplicate inline function
> > and then clean up by putting it into some header file (and maybe hunting down
> > other places where it could be used).

Sounds like a good plan.

> Yeah, I copy/paste cobbled that together. I figured it was easier to
> 'borrow' something that worked and adapt it than try and write
> something new in a hurry.

Yeah.

> > Also some PeterZ code. As the comment implies we really shouldn't be able
> > to get here. This whole function should only be called on CPU models that
> > support the MSR ... but PeterZ is defending against the situation that sometimes
> > there are special SKUs with the same model number (since we may be here because
> > of an x86_match_cpu() hit, rather than the architectural enumeration check).
> 
> My thinking was Virt, virt likes to mess up all msr expectations.

My only worry is to have it written down why we're doing this so that it
can be changed/removed later, when we've forgotten all about split lock.
Because pretty often we look at a comment-less chunk of code and wonder,
"why the hell did we add this in the first place."

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette