[MODERATED] Re: [PATCH] taa fix

On 2019-11-06 20:26:46, speck for Josh Poimboeuf wrote:
> From: Josh Poimboeuf <[email protected]>
> Subject: [PATCH] x86/speculation/taa: Fix TAA_MSG_SMT printk
> 
> For new IBRS_ALL CPUs, the Enhanced IBRS check at the beginning of
> cpu_bugs_smt_update() causes the function to return early,
> unintentionally skipping the MDS and TAA logic.
> 
> I don't think it's a problem for MDS, because AFAICT there appears to be
> no overlap between IBRS_ALL and MDS-affected CPUs.  So the MDS
> mitigation would be disabled and nothing would need to be done in this
> function anyway.
> 
> But for TAA, the TAA_MSG_SMT string will never get printed on Cascade
> Lake and newer.
> 
> The check is superfluous anyway: when 'spectre_v2_enabled' is
> SPECTRE_V2_IBRS_ENHANCED, 'spectre_v2_user' is always
> SPECTRE_V2_USER_NONE, and so the 'spectre_v2_user' switch statement
> handles it appropriately by doing nothing.  So just remove the check.
> 
> Fixes: 1b42f017415b ("x86/speculation/taa: Add mitigation for TSX Async Abort")
> Signed-off-by: Josh Poimboeuf <[email protected]>

Reviewed-by: Tyler Hicks <[email protected]>

I went back through my testing logs and verified that I didn't see the
TAA_MSG_SMT message printed on the Cascade Lake machine I tested with.
Thanks for noticing and fixing it.

Tyler

> ---
>  arch/x86/kernel/cpu/bugs.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 850005590167..4c7b0fa15a19 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -886,10 +886,6 @@ static void update_mds_branch_idle(void)
>  
>  void cpu_bugs_smt_update(void)
>  {
> -	/* Enhanced IBRS implies STIBP. No update required. */
> -	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
> -		return;
> -
>  	mutex_lock(&spec_ctrl_mutex);
>  
>  	switch (spectre_v2_user) {
> -- 
> 2.20.1
>