Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Sun, 7 Aug 2022 12:35:03 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Dave Hansen <dave.hansen@intel.com>, ira.weiny@intel.com,
        Rik van Riel <riel@surriel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry
Message-ID: <Yu+VV8xFTYRh0D0a@zn.tnic>
References: <20220805173009.3128098-1-ira.weiny@intel.com>
 <20220805173009.3128098-6-ira.weiny@intel.com>
 <5d62c1d0-7425-d5bb-ecb5-1dc3b4d7d245@intel.com>
 <Yu4t0jkpIY4grgxL@gmail.com>
 <Yu4wON0MRGH7h5Jv@zn.tnic>
 <Yu+Nwbn4CZUmyD14@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <Yu+Nwbn4CZUmyD14@gmail.com>
Precedence: bulk

On Sun, Aug 07, 2022 at 12:02:41PM +0200, Ingo Molnar wrote:
> * Borislav Petkov <bp@alien8.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().

> - 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