Re: [PATCH v3 3/7] KVM: Add basic bitmap support into kvm_mmu_notifier_test/clear_young

[James Houghton <jthoughton@google.com>]

On Fri, Apr 12, 2024 at 1:28 PM David Matlack <dmatlack@google.com> wrote:
>
> On 2024-04-01 11:29 PM, James Houghton wrote:
> > Add kvm_arch_prepare_bitmap_age() for architectures to indiciate that
> > they support bitmap-based aging in kvm_mmu_notifier_test_clear_young()
> > and that they do not need KVM to grab the MMU lock for writing. This
> > function allows architectures to do other locking or other preparatory
> > work that it needs.
>
> There's a lot going on here. I know it's extra work but I think the
> series would be easier to understand and simplify if you introduced the
> KVM support for lockless test/clear_young() first, and then introduce
> support for the bitmap-based look-around.

Yeah I think this is the right thing to do. Thanks.

>
> Specifically:
>
>  1. Make all test/clear_young() notifiers lockless. i.e. Move the
>     mmu_lock into the architecture-specific code (kvm_age_gfn() and
>     kvm_test_age_gfn()).
>
>  2. Convert KVM/x86's kvm_{test,}_age_gfn() to be lockless for the TDP
>     MMU.
>
>  4. Convert KVM/arm64's kvm_{test,}_age_gfn() to hold the mmu_lock in
>     read-mode.
>
>  5. Add bitmap-based look-around support to KVM/x86 and KVM/arm64
>     (probably 2-3 patches).

This all sounds good to me. Thanks for laying it out for me -- this
should be a lot simpler.

>
> >
> > If an architecture does not implement kvm_arch_prepare_bitmap_age() or
> > is unable to do bitmap-based aging at runtime (and marks the bitmap as
> > unreliable):
> >  1. If a bitmap was provided, we inform the caller that the bitmap is
> >     unreliable (MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE).
> >  2. If a bitmap was not provided, fall back to the old logic.
> >
> > Also add logic for architectures to easily use the provided bitmap if
> > they are able. The expectation is that the architecture's implementation
> > of kvm_gfn_test_age() will use kvm_gfn_record_young(), and
> > kvm_gfn_age() will use kvm_gfn_should_age().
> >
> > Suggested-by: Yu Zhao <yuzhao@google.com>
> > Signed-off-by: James Houghton <jthoughton@google.com>
> > ---
> >  include/linux/kvm_host.h | 60 ++++++++++++++++++++++++++
> >  virt/kvm/kvm_main.c      | 92 +++++++++++++++++++++++++++++-----------
> >  2 files changed, 127 insertions(+), 25 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 1800d03a06a9..5862fd7b5f9b 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1992,6 +1992,26 @@ extern const struct _kvm_stats_desc kvm_vm_stats_desc[];
> >  extern const struct kvm_stats_header kvm_vcpu_stats_header;
> >  extern const struct _kvm_stats_desc kvm_vcpu_stats_desc[];
> >
> > +/*
> > + * Architectures that support using bitmaps for kvm_age_gfn() and
> > + * kvm_test_age_gfn should return true for kvm_arch_prepare_bitmap_age()
> > + * and do any work they need to prepare. The subsequent walk will not
> > + * automatically grab the KVM MMU lock, so some architectures may opt
> > + * to grab it.
> > + *
> > + * If true is returned, a subsequent call to kvm_arch_finish_bitmap_age() is
> > + * guaranteed.
> > + */
> > +#ifndef kvm_arch_prepare_bitmap_age
> > +static inline bool kvm_arch_prepare_bitmap_age(struct mmu_notifier *mn)
>
> I find the name of these architecture callbacks misleading/confusing.
> The lockless path is used even when a bitmap is not provided. i.e.
> bitmap can be NULL in between kvm_arch_prepare/finish_bitmap_age().

Yes. I am really terrible at picking names.... I'm happy to just nix
this, following your other suggestions.

>
> > +{
> > +     return false;
> > +}
> > +#endif
> > +#ifndef kvm_arch_finish_bitmap_age
> > +static inline void kvm_arch_finish_bitmap_age(struct mmu_notifier *mn) {}
> > +#endif
>
> kvm_arch_finish_bitmap_age() seems unnecessary. I think the KVM/arm64
> code could acquire/release the mmu_lock in read-mode in
> kvm_test_age_gfn() and kvm_age_gfn() right?

Yes you're right, except that the way it is now, we only lock/unlock
once for the notifier instead of once for each overlapping memslot,
but that's not an issue, as you mention below.

>
> > +
> >  #ifdef CONFIG_KVM_GENERIC_MMU_NOTIFIER
> >  static inline struct kvm *mmu_notifier_to_kvm(struct mmu_notifier *mn)
> >  {
> > @@ -2076,9 +2096,16 @@ static inline bool mmu_invalidate_retry_gfn_unsafe(struct kvm *kvm,
> >       return READ_ONCE(kvm->mmu_invalidate_seq) != mmu_seq;
> >  }
> >
> > +struct test_clear_young_metadata {
> > +     unsigned long *bitmap;
> > +     unsigned long bitmap_offset_end;
>
> bitmap_offset_end is unused.

Indeed, sorry about that.

>
> > +     unsigned long end;
> > +     bool unreliable;
> > +};
> >  union kvm_mmu_notifier_arg {
> >       pte_t pte;
> >       unsigned long attributes;
> > +     struct test_clear_young_metadata *metadata;
>
> nit: Maybe s/metadata/test_clear_young/ ?

Yes, that's better.

>
> >  };
> >
> >  struct kvm_gfn_range {
> > @@ -2087,11 +2114,44 @@ struct kvm_gfn_range {
> >       gfn_t end;
> >       union kvm_mmu_notifier_arg arg;
> >       bool may_block;
> > +     bool lockless;
>
> Please document this as it's somewhat subtle. A reader might think this
> implies the entire operation runs without taking the mmu_lock.

Will do, and I'll improve the comments for the other "lockless"
variables. (In fact, it might be better to rename/adjust this one to
"mmu_lock_taken" instead -- it's a little more obvious what that
means.)

>
> >  };
> >  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);
> >  bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range);
> >  bool kvm_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range);
> > +
> > +static inline void kvm_age_set_unreliable(struct kvm_gfn_range *range)
> > +{
> > +     struct test_clear_young_metadata *args = range->arg.metadata;
> > +
> > +     args->unreliable = true;
> > +}
> > +static inline unsigned long kvm_young_bitmap_offset(struct kvm_gfn_range *range,
> > +                                                 gfn_t gfn)
> > +{
> > +     struct test_clear_young_metadata *args = range->arg.metadata;
> > +
> > +     return hva_to_gfn_memslot(args->end - 1, range->slot) - gfn;
> > +}
> > +static inline void kvm_gfn_record_young(struct kvm_gfn_range *range, gfn_t gfn)
> > +{
> > +     struct test_clear_young_metadata *args = range->arg.metadata;
> > +
> > +     WARN_ON_ONCE(gfn < range->start || gfn >= range->end);
> > +     if (args->bitmap)
> > +             __set_bit(kvm_young_bitmap_offset(range, gfn), args->bitmap);
> > +}
> > +static inline bool kvm_gfn_should_age(struct kvm_gfn_range *range, gfn_t gfn)
> > +{
> > +     struct test_clear_young_metadata *args = range->arg.metadata;
> > +
> > +     WARN_ON_ONCE(gfn < range->start || gfn >= range->end);
> > +     if (args->bitmap)
> > +             return test_bit(kvm_young_bitmap_offset(range, gfn),
> > +                             args->bitmap);
> > +     return true;
> > +}
> >  #endif
> >
> >  #ifdef CONFIG_HAVE_KVM_IRQ_ROUTING
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index d0545d88c802..7d80321e2ece 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -550,6 +550,7 @@ struct kvm_mmu_notifier_range {
> >       on_lock_fn_t on_lock;
> >       bool flush_on_ret;
> >       bool may_block;
> > +     bool lockless;
> >  };
> >
> >  /*
> > @@ -598,6 +599,8 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
> >       struct kvm_memslots *slots;
> >       int i, idx;
> >
> > +     BUILD_BUG_ON(sizeof(gfn_range.arg) != sizeof(gfn_range.arg.pte));
> > +
> >       if (WARN_ON_ONCE(range->end <= range->start))
> >               return r;
> >
> > @@ -637,15 +640,18 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
> >                       gfn_range.start = hva_to_gfn_memslot(hva_start, slot);
> >                       gfn_range.end = hva_to_gfn_memslot(hva_end + PAGE_SIZE - 1, slot);
> >                       gfn_range.slot = slot;
> > +                     gfn_range.lockless = range->lockless;
> >
> >                       if (!r.found_memslot) {
> >                               r.found_memslot = true;
> > -                             KVM_MMU_LOCK(kvm);
> > -                             if (!IS_KVM_NULL_FN(range->on_lock))
> > -                                     range->on_lock(kvm);
> > -
> > -                             if (IS_KVM_NULL_FN(range->handler))
> > -                                     break;
> > +                             if (!range->lockless) {
> > +                                     KVM_MMU_LOCK(kvm);
> > +                                     if (!IS_KVM_NULL_FN(range->on_lock))
> > +                                             range->on_lock(kvm);
> > +
> > +                                     if (IS_KVM_NULL_FN(range->handler))
> > +                                             break;
> > +                             }
> >                       }
> >                       r.ret |= range->handler(kvm, &gfn_range);
> >               }
> > @@ -654,7 +660,7 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
> >       if (range->flush_on_ret && r.ret)
> >               kvm_flush_remote_tlbs(kvm);
> >
> > -     if (r.found_memslot)
> > +     if (r.found_memslot && !range->lockless)
> >               KVM_MMU_UNLOCK(kvm);
> >
> >       srcu_read_unlock(&kvm->srcu, idx);
> > @@ -682,19 +688,24 @@ static __always_inline int kvm_handle_hva_range(struct mmu_notifier *mn,
> >       return __kvm_handle_hva_range(kvm, &range).ret;
> >  }
> >
> > -static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn,
> > -                                                      unsigned long start,
> > -                                                      unsigned long end,
> > -                                                      gfn_handler_t handler)
> > +static __always_inline int kvm_handle_hva_range_no_flush(
> > +             struct mmu_notifier *mn,
> > +             unsigned long start,
> > +             unsigned long end,
> > +             gfn_handler_t handler,
> > +             union kvm_mmu_notifier_arg arg,
> > +             bool lockless)
> >  {
> >       struct kvm *kvm = mmu_notifier_to_kvm(mn);
> >       const struct kvm_mmu_notifier_range range = {
> >               .start          = start,
> >               .end            = end,
> >               .handler        = handler,
> > +             .arg            = arg,
> >               .on_lock        = (void *)kvm_null_fn,
> >               .flush_on_ret   = false,
> >               .may_block      = false,
> > +             .lockless       = lockless,
> >       };
> >
> >       return __kvm_handle_hva_range(kvm, &range).ret;
> > @@ -909,15 +920,36 @@ static int kvm_mmu_notifier_clear_flush_young(struct mmu_notifier *mn,
> >                                   kvm_age_gfn);
> >  }
> >
> > -static int kvm_mmu_notifier_clear_young(struct mmu_notifier *mn,
> > -                                     struct mm_struct *mm,
> > -                                     unsigned long start,
> > -                                     unsigned long end,
> > -                                     unsigned long *bitmap)
> > +static int kvm_mmu_notifier_test_clear_young(struct mmu_notifier *mn,
> > +                                          struct mm_struct *mm,
> > +                                          unsigned long start,
> > +                                          unsigned long end,
> > +                                          unsigned long *bitmap,
> > +                                          bool clear)
>
> Perhaps pass in the callback (kvm_test_age_gfn/kvm_age_gfn) instead of
> true/false to avoid the naked booleans at the callsites?

Will do. Thank you.

>
> >  {
> > -     trace_kvm_age_hva(start, end);
> > +     if (kvm_arch_prepare_bitmap_age(mn)) {
> > +             struct test_clear_young_metadata args = {
> > +                     .bitmap         = bitmap,
> > +                     .end            = end,
> > +                     .unreliable     = false,
> > +             };
> > +             union kvm_mmu_notifier_arg arg = {
> > +                     .metadata = &args
> > +             };
> > +             bool young;
> > +
> > +             young = kvm_handle_hva_range_no_flush(
> > +                                     mn, start, end,
> > +                                     clear ? kvm_age_gfn : kvm_test_age_gfn,
> > +                                     arg, true);
>
> I suspect the end result will be cleaner we make all architectures
> lockless. i.e. Move the mmu_lock acquire/release into the
> architecture-specific code.
>
> This could result in more acquire/release calls (one per memslot that
> overlaps the provided range) but that should be a single memslot in the
> majority of cases I think?
>
> Then unconditionally pass in the metadata structure.
>
> Then you don't need any special casing for the fast path / bitmap path.
> The only thing needed is to figure out whether to return
> MMU_NOTIFIER_YOUNG vs MMU_NOTIFIER_YOUNG_LOOK_AROUND and that can be
> plumbed via test_clear_young_metadata or by changing gfn_handler_t to
> return an int instead of a bool.

Yes I think this simplification is a great idea. I agree that usually
there will only be one memslot that overlaps a virtual address range
in practice (MIN_LRU_BATCH is BITS_PER_LONG), so the theoretical
additional locking/unlocking shouldn't be an issue.

>
> > +
> > +             kvm_arch_finish_bitmap_age(mn);
> >
> > -     /* We don't support bitmaps. Don't test or clear anything. */
> > +             if (!args.unreliable)
> > +                     return young ? MMU_NOTIFIER_YOUNG_FAST : 0;
> > +     }
> > +
> > +     /* A bitmap was passed but the architecture doesn't support bitmaps */
> >       if (bitmap)
> >               return MMU_NOTIFIER_YOUNG_BITMAP_UNRELIABLE;
> >
> > @@ -934,7 +966,21 @@ static int kvm_mmu_notifier_clear_young(struct mmu_notifier *mn,
> >        * cadence. If we find this inaccurate, we might come up with a
> >        * more sophisticated heuristic later.
> >        */
> > -     return kvm_handle_hva_range_no_flush(mn, start, end, kvm_age_gfn);
> > +     return kvm_handle_hva_range_no_flush(
> > +                     mn, start, end, clear ? kvm_age_gfn : kvm_test_age_gfn,
> > +                     KVM_MMU_NOTIFIER_NO_ARG, false);
>
> Should this return MMU_NOTIFIER_YOUNG explicitly? This code is assuming
> MMU_NOTIFIER_YOUNG == (int)true.

Yes.

Thank you for all the feedback!

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