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

[Andrea Arcangeli <aarcange@redhat.com>]

Hi Thomas,

On Fri, Mar 01, 2019 at 10:47:48PM +0100, speck for Thomas Gleixner wrote:
> +/* 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, so clearing the buffers on
> +	 * idle would be a window dressing exercise.
> +	 */
> +	if (!boot_cpu_has(X86_BUG_MSBDS_ONLY))
> +		return;
> +
> +	if (sched_smt_active())
> +		static_branch_enable(&mds_idle_clear);

Do you think it's worth also clearing
MSR_MISC_FEATURES_ENABLES_RING3MWAIT_BIT by setting
ring3mwait_disabled when sched_smt_active() is true above?

I don't expect anybody will pass manually ring3mwait=disable to the
kernel on XEON_PHI_KNL/XEON_PHI_KNM. I'm not aware of any app using
the user mwait, which also makes this not a big deal.. but it goes
both ways, it's also not a big deal for userland to turn it off when
we report SMT is enabled and safe in sysfs.

Thanks,
Andrea