linux-kernel - Re: [PATCH v3] x86,mm: print likely CPU at segfault time

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Date:   Sat, 6 Aug 2022 10:59:03 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3] x86,mm: print likely CPU at segfault time


* Ingo Molnar <mingo@...nel.org> wrote:

> >  	instrumentation_begin();
> > -	handle_page_fault(regs, error_code, address);
> > +	handle_page_fault(regs, error_code, address, cpu);
> 
> Not convinced that this is a good change: this will bloat all the 
> affected code by a couple of dozen instructions - for no good reason in 
> the context of this patch.
> 
> Boris, why should we do this? Extracting a parameter at higher levels and 
> passing it down to lower levels is almost always a bad idea from a code 
> generation POV, unless the majority of lower levels needs this 
> information anyway (which isn't the case here).

Oh, I just got to this series in my mbox:

  [RFC PATCH 0/5] Print CPU at segfault time
  ...
  [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry

With that basis, printing the segfault CPU becomes a 'free' feature.

At the cost of putting ~2 new instructions into the hotpath of every 
exception though. :-/

Thanks,

	Ingo