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Date:   Wed, 10 Feb 2021 09:10:35 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     "Schofield, Alison" <alison.schofield@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Igor Mammedov <imammedo@redhat.com>,
        Prarit Bhargava <prarit@redhat.com>,
        "brice.goglin@gmail.com" <brice.goglin@gmail.com>
Subject: Re: [PATCH] x86, sched: Allow NUMA nodes to share an LLC on Intel
 platforms
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References: <20210209223943.9834-1-alison.schofield@intel.com>
 <af770863e70340d294c324fd7004f658@intel.com>
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On Tue, Feb 09, 2021 at 11:09:27PM +0000, Luck, Tony wrote:
> > +#define X86_BUG_NUMA_SHARES_LLC		X86_BUG(25) /* CPU may enumerate an LLC shared by multiple NUMA nodes */
> 
> During internal review I wondered why this is a "BUG" rather than a "FEATURE" bit.
> 
> Apparently, the suggestion for "BUG" came from earlier community discussions.
> 
> Historically it may have seemed reasonable to say that a cache cannot span
> NUMA domains. But with more and more things moving off the motherboard
> and into the socket, this doesn't seem too weird now.

If you look at the details this SNC LLC span doesn't behave quite right
either.

It really isn't a regular cache, but behaves a bit like a mash-up of the
s390 book caches and a normal LLC.

Did anybody play with adding the book domain to these SNC
configurations? Can we detect SNC other than by this quirk?