Re: [PATCH v6 02/11] KVM: x86: Relax locking for kvm_test_age_gfn and kvm_age_gfn

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 29, 2024, James Houghton wrote:
> On Fri, Aug 16, 2024 at 6:05 PM Sean Christopherson <seanjc@google.com> wrote:
> > > +static __always_inline bool kvm_tdp_mmu_handle_gfn_lockless(
> > > +             struct kvm *kvm,
> > > +             struct kvm_gfn_range *range,
> > > +             tdp_handler_t handler)
> >
> > Please burn all the Google3 from your brain, and code ;-)
> 
> I indented this way to avoid going past the 80 character limit. I've
> adjusted it to be more like the other functions in this file.
> 
> Perhaps I should put `static __always_inline bool` on its own line?

Noooo. Do not wrap before the function name.  Linus has a nice explanation/rant
on this[1].

In this case, I'm pretty sure you can avoid the helper and simply handle all aging
paths in a single API, e.g. similar to what I proposed for the shadow MMU[2].

[1] https://lore.kernel.org/all/CAHk-=wjoLAYG446ZNHfg=GhjSY6nFmuB_wA8fYd5iLBNXjo9Bw@mail.gmail.com
[2] https://lore.kernel.org/all/20240809194335.1726916-16-seanjc@google.com

> > >  /*
> > >   * Mark the SPTEs range of GFNs [start, end) unaccessed and return non-zero
> > >   * if any of the GFNs in the range have been accessed.
> > > @@ -1237,28 +1272,30 @@ static bool age_gfn_range(struct kvm *kvm, struct tdp_iter *iter,
> > >  {
> > >       u64 new_spte;
> > >
> > > +retry:
> > >       /* If we have a non-accessed entry we don't need to change the pte. */
> > >       if (!is_accessed_spte(iter->old_spte))
> > >               return false;
> > >
> > >       if (spte_ad_enabled(iter->old_spte)) {
> > > -             iter->old_spte = tdp_mmu_clear_spte_bits(iter->sptep,
> > > -                                                      iter->old_spte,
> > > -                                                      shadow_accessed_mask,
> > > -                                                      iter->level);
> > > +             iter->old_spte = tdp_mmu_clear_spte_bits_atomic(iter->sptep,
> > > +                                             shadow_accessed_mask);
> > >               new_spte = iter->old_spte & ~shadow_accessed_mask;
> > >       } else {
> > > -             /*
> > > -              * Capture the dirty status of the page, so that it doesn't get
> > > -              * lost when the SPTE is marked for access tracking.
> > > -              */
> > > +             new_spte = mark_spte_for_access_track(iter->old_spte);
> > > +             if (__tdp_mmu_set_spte_atomic(iter, new_spte)) {
> > > +                     /*
> > > +                      * The cmpxchg failed. If the spte is still a
> > > +                      * last-level spte, we can safely retry.
> > > +                      */
> > > +                     if (is_shadow_present_pte(iter->old_spte) &&
> > > +                         is_last_spte(iter->old_spte, iter->level))
> > > +                             goto retry;
> >
> > Do we have a feel for how often conflicts actually happen?  I.e. is it worth
> > retrying and having to worry about infinite loops, however improbable they may
> > be?
> 
> I'm not sure how common this is. I think it's probably better not to
> retry actually. If the cmpxchg fails, this spte is probably young
> anyway, so I can just `return true` instead of potentially retrying.
> This is all best-effort anyway.

+1