[MODERATED] Re: [patch V6 10/14] MDS basics 10

On Fri, Mar 01, 2019 at 10:47:48PM +0100, speck for Thomas Gleixner wrote:
> Subject: [patch V6 10/14] x86/speculation/mds: Add mitigation control for MDS
> From: Thomas Gleixner <[email protected]>
> 
> Now that the mitigations are in place, add a command line parameter to
> control the mitigation, a mitigation selector function and a SMT update
> mechanism.
> 
> This is the minimal straight forward initial implementation which just
> provides an always on/off mode. The command line parameter is:
> 
>   mds=[full|off]
> 
> This is consistent with the existing mitigations for other speculative
> hardware vulnerabilities.
> 
> The idle invocation is dynamically updated according to the SMT state of
> the system similar to the dynamic update of the STIBP mitigation. The idle
> mitigation is limited to CPUs which are only affected by MSBDS and not any
> other variant, because the other variants cannot be mitigated on SMT
> enabled systems.
> 
> Signed-off-by: Thomas Gleixner <[email protected]>
> ---
> V5 --> V6: Make idle clearing depend on BUG_MSBDS_ONLY
> V4 --> V5: Remove 'auto'
> ---

...

> @@ -617,6 +664,24 @@ static void update_indir_branch_cond(voi
>  		static_branch_disable(&switch_to_cond_stibp);
>  }
>  
> +/* Update the static key controlling the MDS CPU buffer clear in idle */
> +static void update_mds_branch_idle(void)
> +{
> +	/*
> +	 * Enable the idle clearing on CPUs which are affected only by
> +	 * MDBDS and not any other MDS variant. The other variants cannot
> +	 * be mitigated when SMT is enabled,

... but we're not enabling the key when SMT on those is disabled,
AFAICT. Or is that coming later?

> so clearing the buffers on
> +	 * idle would be a window dressing exercise.
> +	 */
> +	if (!boot_cpu_has(X86_BUG_MSBDS_ONLY))

	if (!boot_cpu_has_bug

> +		return;
> +
> +	if (sched_smt_active())
> +		static_branch_enable(&mds_idle_clear);
> +	else
> +		static_branch_disable(&mds_idle_clear);
> +}
> +
>  void arch_smt_update(void)
>  {
>  	/* Enhanced IBRS implies STIBP. No update required. */
> @@ -638,6 +703,9 @@ void arch_smt_update(void)
>  		break;
>  	}
>  
> +	if (mds_mitigation == MDS_MITIGATION_FULL)
> +		update_mds_branch_idle();
> +
>  	mutex_unlock(&spec_ctrl_mutex);
>  }
>  
> 

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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