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Date:   Wed, 21 Jun 2023 19:05:41 -0700
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Subject: Re: [PATCH v3 1/3] x86/idle: Disable IBRS when cpu is offline
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To:     Waiman Long <longman@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Len Brown <lenb@kernel.org>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-pm@vger.kernel.org, Robin Jarry <rjarry@redhat.com>,
        Joe Mario <jmario@redhat.com>
References: <20230622003603.1188364-1-longman@redhat.com>
 <20230622003603.1188364-2-longman@redhat.com>
From:   Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <20230622003603.1188364-2-longman@redhat.com>
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On 6/21/23 17:36, Waiman Long wrote:
> Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
> disables IBRS when the CPU enters long idle. However, when a CPU
> becomes offline, the IBRS bit is still set when X86_FEATURE_KERNEL_IBRS
> is enabled. That will impact the performance of a sibling CPU. Mitigate
> this performance impact by clearing all the mitigation bits in SPEC_CTRL
> MSR when offline. When the CPU is online again, it will be re-initialized
> and so restoring the SPEC_CTRL value isn't needed.
> 
> Add a comment to say that native_play_dead() is a __noreturn function,
> but it can't be marked as such to avoid confusion about the missing
> MSR restoration code.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  arch/x86/kernel/smpboot.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 352f0ce1ece4..7bc33885518c 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -84,6 +84,7 @@
>  #include <asm/hw_irq.h>
>  #include <asm/stackprotector.h>
>  #include <asm/sev.h>
> +#include <asm/nospec-branch.h>
>  
>  /* representing HT siblings of each logical CPU */
>  DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);
> @@ -1836,8 +1837,17 @@ void __noreturn hlt_play_dead(void)
>  	}
>  }
>  
> +/*
> + * naitve_play_dead() is essentially a __noreturn function, but it can't

typo: native_play_dead()

> + * be marked as such as the compiler may complain about it.
> + */
>  void native_play_dead(void)
>  {
> +	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
> +		this_cpu_write(x86_spec_ctrl_current, 0);
> +		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> +	}
> +
>  	play_dead_common();
>  	tboot_shutdown(TB_SHUTDOWN_WFS);
>  

-- 
~Randy