Re: [PATCH v5 3/9] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age_gfn

[James Houghton <jthoughton@google.com>]

On Mon, Jun 10, 2024 at 10:58 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> On Tue, Jun 11, 2024 at 12:21:39AM +0000, James Houghton wrote:
> > Replace the MMU write locks (taken in the memslot iteration loop) for
> > read locks.
> >
> > Grabbing the read lock instead of the write lock is safe because the
> > only requirement we have is that the stage-2 page tables do not get
> > deallocated while we are walking them. The stage2_age_walker() callback
> > is safe to race with itself; update the comment to reflect the
> > synchronization change.
> >
> > Signed-off-by: James Houghton <jthoughton@google.com>
> > ---
> >  arch/arm64/kvm/Kconfig       |  1 +
> >  arch/arm64/kvm/hyp/pgtable.c | 15 +++++++++------
> >  arch/arm64/kvm/mmu.c         | 26 ++++++++++++++++++++------
> >  3 files changed, 30 insertions(+), 12 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> > index 58f09370d17e..7a1af8141c0e 100644
> > --- a/arch/arm64/kvm/Kconfig
> > +++ b/arch/arm64/kvm/Kconfig
> > @@ -22,6 +22,7 @@ menuconfig KVM
> >       select KVM_COMMON
> >       select KVM_GENERIC_HARDWARE_ENABLING
> >       select KVM_GENERIC_MMU_NOTIFIER
> > +     select KVM_MMU_NOTIFIER_YOUNG_LOCKLESS
> >       select HAVE_KVM_CPU_RELAX_INTERCEPT
> >       select KVM_MMIO
> >       select KVM_GENERIC_DIRTYLOG_READ_PROTECT
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 9e2bbee77491..b1b0f7148cff 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -1319,10 +1319,10 @@ static int stage2_age_walker(const struct kvm_pgtable_visit_ctx *ctx,
> >       data->young = true;
> >
> >       /*
> > -      * stage2_age_walker() is always called while holding the MMU lock for
> > -      * write, so this will always succeed. Nonetheless, this deliberately
> > -      * follows the race detection pattern of the other stage-2 walkers in
> > -      * case the locking mechanics of the MMU notifiers is ever changed.
> > +      * This walk may not be exclusive; the PTE is permitted to change
>
> s/may not/is not/

Will fix.

>
> > +      * from under us. If there is a race to update this PTE, then the
> > +      * GFN is most likely young, so failing to clear the AF is likely
> > +      * to be inconsequential.
> >        */
> >       if (data->mkold && !stage2_try_set_pte(ctx, new))
> >               return -EAGAIN;
> > @@ -1345,10 +1345,13 @@ bool kvm_pgtable_stage2_test_clear_young(struct kvm_pgtable *pgt, u64 addr,
> >       struct kvm_pgtable_walker walker = {
> >               .cb             = stage2_age_walker,
> >               .arg            = &data,
> > -             .flags          = KVM_PGTABLE_WALK_LEAF,
> > +             .flags          = KVM_PGTABLE_WALK_LEAF |
> > +                               KVM_PGTABLE_WALK_SHARED,
> >       };
> > +     int r;
> >
> > -     WARN_ON(kvm_pgtable_walk(pgt, addr, size, &walker));
> > +     r = kvm_pgtable_walk(pgt, addr, size, &walker);
> > +     WARN_ON(r && r != -EAGAIN);
>
> I could've been more explicit last time around, could you please tone
> this down to WARN_ON_ONCE() as well?

Will do, thanks.

>
> >       return data.young;
> >  }
> >
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 8bcab0cc3fe9..a62c27a347ed 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -1773,25 +1773,39 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
> >  bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> >  {
> >       u64 size = (range->end - range->start) << PAGE_SHIFT;
> > +     bool young = false;
> > +
> > +     read_lock(&kvm->mmu_lock);
> >
> >       if (!kvm->arch.mmu.pgt)
> >               return false;
>
> I'm guessing you meant to have 'goto out' here, since this early return
> fails to drop the mmu_lock.

Ah sorry! Thanks for catching this.