Re: [Patch v3 04/13] x86/speculation: Disable STIBP when enhanced IBRS is in use

[Waiman Long <longman9394@gmail.com>]

On 10/17/2018 01:59 PM, Tim Chen wrote:
> With enhanced IBRS in use, the application running on sibling CPU will not
> be able to launch Spectre v2 attack to the application on current CPU.
> There is no need to use STIBP for this case.  Disable the STIBP code
> when enhanced IBRS is used.
>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 2fc7b4e..6ed82ea 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -327,6 +327,14 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
>  
>  static bool stibp_needed(void)
>  {
> +	/*
> +	 * Determine if we want to leave STIBP always on.
> +	 * Using enhanced IBRS makes using STIBP unnecessary.
> +	 */
> +
> +	if (static_branch_unlikely(&spectre_v2_enhanced_ibrs))
> +		return false;
> +
>  	if (spectre_v2_enabled == SPECTRE_V2_NONE)
>  		return false;
>  
> @@ -881,7 +889,9 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr
>  				   ", IBPB" : "",
>  			boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ?
>  				   ", IBRS_FW" : "",
> -			(x86_spec_ctrl_base & SPEC_CTRL_STIBP) ?
> +			spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED ?
> +				   ", Enhanced IBRS" :
> +			    (x86_spec_ctrl_base & SPEC_CTRL_STIBP) ?
>  				   ", STIBP" : "",
>  			boot_cpu_has(X86_FEATURE_RSB_CTXSW) ?
>  				   ", RSB filling" : "",

The "Enhanced IBRS" is one of the states of spectre_v2_enabled. So you
don't need to print that out one more time.

Cheers,
Longman