linux-kernel - Re: [PATCH v3 3/8] x86/split_lock: Cache the value of MSR_TEST

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Date:   Thu, 6 Feb 2020 15:23:47 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        hpa@...or.com, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>, tony.luck@...el.com,
        peterz@...radead.org, fenghua.yu@...el.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/8] x86/split_lock: Cache the value of MSR_TEST_CTRL
 in percpu data

On Thu, Feb 06, 2020 at 03:04:07PM +0800, Xiaoyao Li wrote:
> Cache the value of MSR_TEST_CTRL in percpu data msr_test_ctrl_cache,
> which will be used by KVM module.
> 
> It also avoids an expensive RDMSR instruction if SLD needs to be context
> switched.
> 
> Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> ---
>  arch/x86/include/asm/cpu.h  |  2 ++
>  arch/x86/kernel/cpu/intel.c | 19 ++++++++++++-------
>  2 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
> index ff567afa6ee1..2b20829db450 100644
> --- a/arch/x86/include/asm/cpu.h
> +++ b/arch/x86/include/asm/cpu.h
> @@ -27,6 +27,8 @@ struct x86_cpu {
>  };
>  
>  #ifdef CONFIG_HOTPLUG_CPU
> +DECLARE_PER_CPU(u64, msr_test_ctrl_cache);
> +

Why does this depend on HOTPLUG_CPU?