linux-kernel - Re: [PATCH v2] x86,mm: print likely CPU at segfault time

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Message-ID: <Yu0aYIbCAjFtEBVZ@zn.tnic>
Date:   Fri, 5 Aug 2022 15:25:52 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Rik van Riel <riel@...riel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Thomas Gleixner <tglx@...utronix.de>, Dave Jones <dsj@...com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2]  x86,mm: print likely CPU at segfault time

On Fri, Aug 05, 2022 at 08:53:46AM -0400, Rik van Riel wrote:
> Having a small percentage of the segfaults show up on
> cores other than the broken one does not cause issues with
> detection or diagnosis.

I'm sorry but I'm not buying any of this: this should either be 100%
correct or it can stay on your kernels.

> We could, but then we would be reading the CPU number
> on every page fault, just in case it's a segfault.
> 
> That does not seem like a worthwhile tradeoff, given
> how much of a hot path page faults are, and how rare
> segfaults are.

Oh wow, a whopping single instruction:

	movl %gs:cpu_number(%rip), %eax # cpu_number, pfo_val__

What tradeoff?

-- 
Regards/Gruss,
    Boris.

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