[PATCH v3 5/7] KVM: x86: Participate in bitmap-based PTE aging

[James Houghton]

On Fri, Apr 19, 2024 at 2:07 PM David Matlack <dmatlack at google.com> wrote:
>
> On 2024-04-19 01:47 PM, James Houghton wrote:
> > On Thu, Apr 11, 2024 at 10:28 AM David Matlack <dmatlack at google.com> wrote:
> > > On 2024-04-11 10:08 AM, David Matlack wrote:
> > > bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> > > {
> > >         bool young = false;
> > >
> > >         if (!range->arg.metadata->bitmap && kvm_memslots_have_rmaps(kvm))
> > >                 young = kvm_handle_gfn_range(kvm, range, kvm_age_rmap);
> > >
> > >         if (tdp_mmu_enabled)
> > >                 young |= kvm_tdp_mmu_age_gfn_range(kvm, range);
> > >
> > >         return young;
> > > }
> > >
> > > bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> > > {
> > >         bool young = false;
> > >
> > >         if (!range->arg.metadata->bitmap && kvm_memslots_have_rmaps(kvm))
> > >                 young = kvm_handle_gfn_range(kvm, range, kvm_test_age_rmap);
> > >
> > >         if (tdp_mmu_enabled)
> > >                 young |= kvm_tdp_mmu_test_age_gfn(kvm, range);
> > >
> > >         return young;
> >
> >
> > Yeah I think this is the right thing to do. Given your other
> > suggestions (on patch 3), I think this will look something like this
> > -- let me know if I've misunderstood something:
> >
> > bool check_rmap = !bitmap && kvm_memslot_have_rmaps(kvm);
> >
> > if (check_rmap)
> >   KVM_MMU_LOCK(kvm);
> >
> > rcu_read_lock(); // perhaps only do this when we don't take the MMU lock?
> >
> > if (check_rmap)
> >   kvm_handle_gfn_range(/* ... */ kvm_test_age_rmap)
> >
> > if (tdp_mmu_enabled)
> >   kvm_tdp_mmu_test_age_gfn() // modified to be RCU-safe
> >
> > rcu_read_unlock();
> > if (check_rmap)
> >   KVM_MMU_UNLOCK(kvm);
>
> I was thinking a little different. If you follow my suggestion to first
> make the TDP MMU aging lockless, you'll end up with something like this
> prior to adding bitmap support (note: the comments are just for
> demonstrative purposes):
>
> bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> {
>         bool young = false;
>
>         /* Shadow MMU aging holds write-lock. */
>         if (kvm_memslots_have_rmaps(kvm)) {
>                 write_lock(&kvm->mmu_lock);
>                 young = kvm_handle_gfn_range(kvm, range, kvm_age_rmap);
>                 write_unlock(&kvm->mmu_lock);
>         }
>
>         /* TDM MMU aging is lockless. */
>         if (tdp_mmu_enabled)
>                 young |= kvm_tdp_mmu_age_gfn_range(kvm, range);
>
>         return young;
> }
>
> Then when you add bitmap support it would look something like this:
>
> bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> {
>         unsigned long *bitmap = range->arg.metadata->bitmap;
>         bool young = false;
>
>         /* SHadow MMU aging holds write-lock and does not support bitmap. */
>         if (kvm_memslots_have_rmaps(kvm) && !bitmap) {
>                 write_lock(&kvm->mmu_lock);
>                 young = kvm_handle_gfn_range(kvm, range, kvm_age_rmap);
>                 write_unlock(&kvm->mmu_lock);
>         }
>
>         /* TDM MMU aging is lockless and supports bitmap. */
>         if (tdp_mmu_enabled)
>                 young |= kvm_tdp_mmu_age_gfn_range(kvm, range);
>
>         return young;
> }
>
> rcu_read_lock/unlock() would be called in kvm_tdp_mmu_age_gfn_range().

Oh yes this is a lot better. I hope I would have seen this when it
came time to actually update this patch. Thanks.

>
> That brings up a question I've been wondering about. If KVM only
> advertises support for the bitmap lookaround when shadow roots are not
> allocated, does that mean MGLRU will be blind to accesses made by L2
> when nested virtualization is enabled? And does that mean the Linux MM
> will think all L2 memory is cold (i.e. good candidate for swapping)
> because it isn't seeing accesses made by L2?

Yes, I think so (for both questions). That's better than KVM not
participating in MGLRU aging at all, which is the case today (IIUC --
also ignoring the case where KVM accesses guest memory directly). We
could have MGLRU always invoke the mmu notifiers, but frequently
taking the MMU lock for writing might be worse than evicting when we
shouldn't. Maybe Yu tried this at some point, but I can't find any
results for this.