linux-kernel - Re: [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry

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Date:   Sun, 7 Aug 2022 13:02:37 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        "Rik van Riel" <riel@...riel.com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry

On Sun, Aug 07, 2022 at 12:35:03PM +0200, Borislav Petkov wrote:
> On Sun, Aug 07, 2022 at 12:02:41PM +0200, Ingo Molnar wrote:
> > * Borislav Petkov <bp@...en8.de> wrote:
> > > With the amount of logical cores ever increasing and how CPU packages
> > > (nodes, L3 sharing, you name it) get more and more complex topology,
> > > I'd say the 2 insns to show the CPU number in every exception is a good
> > > thing to do.
> > 
> > We can show it - I'm arguing against extracting it too early, which costs
> 
> Not early - more correct. We can say which CPU executed the exception
> handler *exactly*. Not which CPU executed the exception handler *maybe*.
> 
> > us 2 instructions in the exception fast path
> 
> 2 insns? They don't matter at all. FWIW, they'll pull in the per-CPU
> cacheline earlier which should be a net win later, for code which does
> smp_processor_id().

I agree with Boris; however I feel that I have to mention that in patch 3/5 you
also have 1 instruction on each of entry and exit to push the extra stack
space.  So all told it would cost 4 instructions.

Again, I don't believe this is too much overhead but I don't want people to say
it was not discussed.

Ira

> 
> > - while in 99.999999999% of the cases we don't use that field at all ...
> 
> See my text above about the ever-increasing complexity of CPU topology.
> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette