linux-kernel - Re: [PATCH] KVM/x86: Increase max vcpu number to 352

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Date:   Sat, 12 Aug 2017 00:47:07 +0200
From:   Denys Vlasenko <vda.linux@...glemail.com>
To:     Lan Tianyu <tianyu.lan@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        KVM list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM/x86: Increase max vcpu number to 352

On Thu, Aug 10, 2017 at 12:00 PM, Lan Tianyu <tianyu.lan@...el.com> wrote:
> Intel Xeon phi chip will support 352 logical threads. For HPC usage
> case, it will create a huge VM with vcpu number as same as host cpus. This
> patch is to increase max vcpu number to 352.

This number was bumped in the past to 288 to accommodate Knights Landing,
but KNL's max designed thread number is actually 304: the on-die
interconnect mesh is 6*7, with four cells taken for interconnect
and memory controllers, there are 38 CPU cells.

Each CPU cell has two cores with shared L2.
Each core is SMT4. 38*8 = 304.

Intel fuses two cells (or more), so 288 is the largest number of threads
on a KNL you can buy, but 304 thread KNLs most probably also exist
(however they may be rather rare since they require completely
defect-free die).

I think it's better if Linux would support those too.

What is the design maximum for these new "nominally 352 thread" Xeon Phis
which are "nominally 352 thread"? 360? (If the mesh is 7*7 and the same
4 cells are taked for non-CPU needs)