On Tue, Jul 17, 2018 at 1:16 PM, Rik van Riel <[email protected]> wrote: > Can I skip both the cr4 and let switches when the TLB contents > are no longer valid and got reloaded? > > If the TLB contents are still valid, either because we never went > into lazy TLB mode, or because no invalidates happened while > we were lazy, we immediately return. > > The cr4 and ldt reloads only happen if the TLB was invalidated > while we were in lazy TLB mode.
Yes, since the only events that would change the LDT or the required CR4 value will unconditionally broadcast to every CPU in mm_cpumask regardless of whether they're lazy. The interesting case is that you go lazy, you miss an invalidation IPI because you were lazy, then you go unlazy, notice the tlb_gen change, and flush. If this happens, you know that you only missed a page table update and not an LDT update or a CR4 update, because the latter would have sent the IPI even though you were lazy. So you should skip the CR4 and LDT updates. I suppose a different approach would be to fix the issue below and to try to track when the LDT actually needs reloading. But that latter part seems a bit complicated for minimal gain. (Do you believe me? If not, please argue back!) >> Hmm. load_mm_cr4() should bypass itself when mm == &init_mm. Want to >> fix that part or should I? > > I would be happy to send in a patch for this, and one for > the above optimization you pointed out. > Yes please!
