Subject: Re: [PATCH mm-unstable v2 07/10] kvm/powerpc: add kvm_arch_test_clear_young()

From: "Nicholas Piggin" <[email-masked]>
Date: Wed, 21 Jun 2023 12:51:31 +1000

On Wed Jun 21, 2023 at 10:38 AM AEST, Yu Zhao wrote:
> On Tue, Jun 20, 2023 at 1:48 AM Nicholas Piggin <[email-masked]> wrote:
> >
> > On Sat May 27, 2023 at 9:44 AM AEST, Yu Zhao wrote:
> > > Implement kvm_arch_test_clear_young() to support the fast path in
> > > mmu_notifier_ops->test_clear_young().
> > >
> > > It focuses on a simple case, i.e., radix MMU sets the accessed bit in
> > > KVM PTEs and VMs are not nested, where it can rely on RCU and
> > > pte_xchg() to safely clear the accessed bit without taking
> > > kvm->mmu_lock. Complex cases fall back to the existing slow path
> > > where kvm->mmu_lock is then taken.
> > >
> > > Signed-off-by: Yu Zhao <[email-masked]>
> > > ---
> > >  arch/powerpc/include/asm/kvm_host.h    |  8 ++++
> > >  arch/powerpc/include/asm/kvm_ppc.h     |  1 +
> > >  arch/powerpc/kvm/book3s.c              |  6 +++
> > >  arch/powerpc/kvm/book3s.h              |  1 +
> > >  arch/powerpc/kvm/book3s_64_mmu_radix.c | 59 ++++++++++++++++++++++++++
> > >  arch/powerpc/kvm/book3s_hv.c           |  5 +++
> > >  6 files changed, 80 insertions(+)
> > >
> > > diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> > > index 14ee0dece853..75c260ea8a9e 100644
> > > --- a/arch/powerpc/include/asm/kvm_host.h
> > > +++ b/arch/powerpc/include/asm/kvm_host.h
> > > @@ -883,4 +883,12 @@ static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
> > >  static inline void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu) {}
> > >  static inline void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu) {}
> > >
> > > +#define kvm_arch_has_test_clear_young kvm_arch_has_test_clear_young
> > > +static inline bool kvm_arch_has_test_clear_young(void)
> > > +{
> > > +     return IS_ENABLED(CONFIG_KVM_BOOK3S_HV_POSSIBLE) &&
> > > +            cpu_has_feature(CPU_FTR_HVMODE) && cpu_has_feature(CPU_FTR_ARCH_300) &&
> > > +            radix_enabled();
> >
> > This could probably be radix_enabled() && !kvmhv_on_pseries().
>
> Will do. (I used !kvmhv_on_pseries() in v1 but had second thoughts on
> moving kvmhv_on_pseries() into this file.)

That should be okay. kvmhv_on_pseries is a property of the host so it
seems reasonable to move it here if needed.

> > Although unclear why not nested hypervisor... I'd have to think about it a bit
> > more. Do you have any idea what might go wrong, or just didn't have the
> > time to consider it? (Not just powerpc nested but any others).
>
> Yes, this series excludes nested KVM support on all architures. The
> common reason for such a decision on powerpc and x86 (aarch64 doesn't
> support nested yet) is that it's quite challenging to make the rmap, a
> complex data structure that maps one PFN to multiple GFNs, lockless.
> (See kvmhv_insert_nest_rmap().) It might be possible, however, the
> potential ROI would be in question.

Okay just wondering. rmap (at least the powerpc one) is just a list
I think, with a few details. If that is all it is, it might not be
so hard to make that lock-free or a fine-grained lock on the rmap
chains maybe. But fine to ignore it to start with.

> > > +}
> > > +
> > >  #endif /* __POWERPC_KVM_HOST_H__ */
> > > diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
> > > index 79a9c0bb8bba..ff1af6a7b44f 100644
> > > --- a/arch/powerpc/include/asm/kvm_ppc.h
> > > +++ b/arch/powerpc/include/asm/kvm_ppc.h
> > > @@ -287,6 +287,7 @@ struct kvmppc_ops {
> > >       bool (*unmap_gfn_range)(struct kvm *kvm, struct kvm_gfn_range *range);
> > >       bool (*age_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);
> > >       bool (*test_age_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);
> > > +     bool (*test_clear_young)(struct kvm *kvm, struct kvm_gfn_range *range);
> > >       bool (*set_spte_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);
> > >       void (*free_memslot)(struct kvm_memory_slot *slot);
> > >       int (*init_vm)(struct kvm *kvm);
> > > diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> > > index 686d8d9eda3e..37bf40b0c4ff 100644
> > > --- a/arch/powerpc/kvm/book3s.c
> > > +++ b/arch/powerpc/kvm/book3s.c
> > > @@ -899,6 +899,12 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> > >       return kvm->arch.kvm_ops->test_age_gfn(kvm, range);
> > >  }
> > >
> > > +bool kvm_arch_test_clear_young(struct kvm *kvm, struct kvm_gfn_range *range)
> > > +{
> > > +     return !kvm->arch.kvm_ops->test_clear_young ||
> > > +            kvm->arch.kvm_ops->test_clear_young(kvm, range);
> > > +}
> > > +
> > >  bool kvm_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> > >  {
> > >       return kvm->arch.kvm_ops->set_spte_gfn(kvm, range);
> > > diff --git a/arch/powerpc/kvm/book3s.h b/arch/powerpc/kvm/book3s.h
> > > index 58391b4b32ed..fa2659e21ccc 100644
> > > --- a/arch/powerpc/kvm/book3s.h
> > > +++ b/arch/powerpc/kvm/book3s.h
> > > @@ -12,6 +12,7 @@ extern void kvmppc_core_flush_memslot_hv(struct kvm *kvm,
> > >  extern bool kvm_unmap_gfn_range_hv(struct kvm *kvm, struct kvm_gfn_range *range);
> > >  extern bool kvm_age_gfn_hv(struct kvm *kvm, struct kvm_gfn_range *range);
> > >  extern bool kvm_test_age_gfn_hv(struct kvm *kvm, struct kvm_gfn_range *range);
> > > +extern bool kvm_test_clear_young_hv(struct kvm *kvm, struct kvm_gfn_range *range);
> > >  extern bool kvm_set_spte_gfn_hv(struct kvm *kvm, struct kvm_gfn_range *range);
> > >
> > >  extern int kvmppc_mmu_init_pr(struct kvm_vcpu *vcpu);
> > > diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
> > > index 3b65b3b11041..0a392e9a100a 100644
> > > --- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
> > > +++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
> > > @@ -1088,6 +1088,65 @@ bool kvm_test_age_radix(struct kvm *kvm, struct kvm_memory_slot *memslot,
> > >       return ref;
> > >  }
> > >
> > > +bool kvm_test_clear_young_hv(struct kvm *kvm, struct kvm_gfn_range *range)
> > > +{
> > > +     bool err;
> > > +     gfn_t gfn = range->start;
> > > +
> > > +     rcu_read_lock();
> > > +
> > > +     err = !kvm_is_radix(kvm);
> > > +     if (err)
> > > +             goto unlock;
> > > +
> > > +     /*
> > > +      * Case 1:  This function          kvmppc_switch_mmu_to_hpt()
> > > +      *
> > > +      *          rcu_read_lock()
> > > +      *          Test kvm_is_radix()    kvm->arch.radix = 0
> > > +      *          Use kvm->arch.pgtable  synchronize_rcu()
> > > +      *          rcu_read_unlock()
> > > +      *                                 kvmppc_free_radix()
> > > +      *
> > > +      *
> > > +      * Case 2:  This function          kvmppc_switch_mmu_to_radix()
> > > +      *
> > > +      *                                 kvmppc_init_vm_radix()
> > > +      *                                 smp_wmb()
> > > +      *          Test kvm_is_radix()    kvm->arch.radix = 1
> > > +      *          smp_rmb()
> > > +      *          Use kvm->arch.pgtable
> > > +      */
> > > +     smp_rmb();
> >
> > Comment could stand to expand slightly on what you are solving, in
> > words.
>
> Will do.
>
> > If you use synchronize_rcu() on both sides, you wouldn't need the
> > barrier, right?
>
> Case 2 is about memory ordering, which is orthogonal to case 1 (RCU
> freeing). So we need the r/w barrier regardless.

RCU can take care of memory ordering too though. If you had
synchronize_rcu() where smp_wmb() is, then no smp_rmb() neeed here.

>
> > > +     while (gfn < range->end) {
> > > +             pte_t *ptep;
> > > +             pte_t old, new;
> > > +             unsigned int shift;
> > > +
> > > +             ptep = find_kvm_secondary_pte_unlocked(kvm, gfn * PAGE_SIZE, &shift);
> > > +             if (!ptep)
> > > +                     goto next;
> > > +
> > > +             VM_WARN_ON_ONCE(!page_count(virt_to_page(ptep)));
> >
> > Not really appropriate at the KVM level. mm enforces this kind of
> > thing (with notifiers).
>
> Will remove this.
>
> > > +
> > > +             old = READ_ONCE(*ptep);
> > > +             if (!pte_present(old) || !pte_young(old))
> > > +                     goto next;
> > > +
> > > +             new = pte_mkold(old);
> > > +
> > > +             if (kvm_should_clear_young(range, gfn))
> > > +                     pte_xchg(ptep, old, new);
> >
> > *Probably* will work. I can't think of a reason why not at the
> > moment anyway :)
>
> My reasoning:
> * It should work if we only change the dedicated A bit, i.e., not
> shared for other purposes, because races are safe (the case here).
> * It may not work for x86 EPT without the A bit (excluded in this
> series) where accesses are trapped by the R/X bits, because races in
> changing the R/X bits can be unsafe.

(For the benefit of others reading, it works because powerpc's pte_xchg
is actually a cmpxchg, for some reason which we really should fix).
Although it can fail to clear the bit if the cmpxchg fails.

I think pte_xchg is only used on with hash MMU in Linux before this
change. I think we may want to keep it that way and use something
like kvmppc_radix_update_pte() here to clear out the bit. But don't
worry too much about fine details so much before sorting out the
core changes I will have a better look after that.

Thanks,
Nick

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