Re: [PATCH v2 2/5] x86/idle: Disable IBRS when cpu is offline

[Peter Zijlstra]

On Tue, Jun 20, 2023 at 10:06:22AM -0400, 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 and restoring the value of the MSR when it becomes
> online again.
> 
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  arch/x86/kernel/smpboot.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 352f0ce1ece4..5ff82fef413c 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);
> @@ -1838,12 +1839,24 @@ void __noreturn hlt_play_dead(void)
>  
>  void native_play_dead(void)
>  {
> +	u64 spec_ctrl = spec_ctrl_current();
> +
> +	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);
>  
>  	mwait_play_dead();
>  	if (cpuidle_play_dead())
>  		hlt_play_dead();
> +
> +	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
> +		native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
> +		this_cpu_write(x86_spec_ctrl_current, spec_ctrl);
> +	}
>  }

play_dead() is marked __noreturn