Re: [PATCH v3 3/5] x86/mm: Optionally flush L1D on context switch

Singh, Balbir
2020-12-04T22:42:40Z

On Fri, 2020-12-04 at 22:21 +0100, Thomas Gleixner wrote:
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> On Fri, Nov 27 2020 at 17:59, Balbir Singh wrote:
> >
> > + /*
> > + * Flush only if SMT is disabled as per the contract, which is checked
> > + * when the feature is enabled.
> > + */
> > + if (sched_smt_active() && !this_cpu_read(cpu_info.smt_active) &&
> > + (prev_mm & LAST_USER_MM_L1D_FLUSH))
> > + l1d_flush_hw();
>
> So if SMT is completely disabled then no flush? Shouldn't the logic be:
>
> if ((!sched_smt_active() || !this_cpu_read(cpu_info.smt_active) &&
> (prev_mm & LAST_USER_MM_L1D_FLUSH))
>
> Hmm?
>
> But that's bad, because it's lot's of conditions to evaluate for every
> switch_mm where most of them are not interested in it at all.
>
> Let me read through the rest of the pile.
>

We don't need this anymore with the new checks for preempting killing
of the task, so it can be removed

Balbir