Re: [PATCH v3 3/8] x86/split_lock: Cache the value of MSR_TEST_CTRL in percpu data

[Arvind Sankar]

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@xxxxxxxxx>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> ---
>  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?