Re: [PATCH v10 6/9] KVM: Unmap existing mappings when change the memory attributes

[Fuad Tabba <tabba@google.com>]

Hi,

On Fri, Dec 2, 2022 at 6:19 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
>
> Unmap the existing guest mappings when memory attribute is changed
> between shared and private. This is needed because shared pages and
> private pages are from different backends, unmapping existing ones
> gives a chance for page fault handler to re-populate the mappings
> according to the new attribute.
>
> Only architecture has private memory support needs this and the
> supported architecture is expected to rewrite the weak
> kvm_arch_has_private_mem().

This kind of ties into the discussion of being able to share memory in
place. For pKVM for example, shared and private memory would have the
same backend, and the unmapping wouldn't be needed.

So I guess that, instead of kvm_arch_has_private_mem(), can the check
be done differently, e.g., with a different function, say
kvm_arch_private_notify_attribute_change() (but maybe with a more
friendly name than what I suggested :) )?

Thanks,
/fuad

>
> Also, during memory attribute changing and the unmapping time frame,
> page fault handler may happen in the same memory range and can cause
> incorrect page state, invoke kvm_mmu_invalidate_* helpers to let the
> page fault handler retry during this time frame.
>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  include/linux/kvm_host.h |   7 +-
>  virt/kvm/kvm_main.c      | 168 ++++++++++++++++++++++++++-------------
>  2 files changed, 116 insertions(+), 59 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 3d69484d2704..3331c0c92838 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -255,7 +255,6 @@ bool kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu);
>  #endif
>
> -#ifdef KVM_ARCH_WANT_MMU_NOTIFIER
>  struct kvm_gfn_range {
>         struct kvm_memory_slot *slot;
>         gfn_t start;
> @@ -264,6 +263,8 @@ struct kvm_gfn_range {
>         bool may_block;
>  };
>  bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range);
> +
> +#ifdef KVM_ARCH_WANT_MMU_NOTIFIER
>  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);
> @@ -785,11 +786,12 @@ struct kvm {
>
>  #if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
>         struct mmu_notifier mmu_notifier;
> +#endif
>         unsigned long mmu_invalidate_seq;
>         long mmu_invalidate_in_progress;
>         gfn_t mmu_invalidate_range_start;
>         gfn_t mmu_invalidate_range_end;
> -#endif
> +
>         struct list_head devices;
>         u64 manual_dirty_log_protect;
>         struct dentry *debugfs_dentry;
> @@ -1480,6 +1482,7 @@ bool kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu);
>  int kvm_arch_post_init_vm(struct kvm *kvm);
>  void kvm_arch_pre_destroy_vm(struct kvm *kvm);
>  int kvm_arch_create_vm_debugfs(struct kvm *kvm);
> +bool kvm_arch_has_private_mem(struct kvm *kvm);
>
>  #ifndef __KVM_HAVE_ARCH_VM_ALLOC
>  /*
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ad55dfbc75d7..4e1e1e113bf0 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -520,6 +520,62 @@ void kvm_destroy_vcpus(struct kvm *kvm)
>  }
>  EXPORT_SYMBOL_GPL(kvm_destroy_vcpus);
>
> +void kvm_mmu_invalidate_begin(struct kvm *kvm)
> +{
> +       /*
> +        * The count increase must become visible at unlock time as no
> +        * spte can be established without taking the mmu_lock and
> +        * count is also read inside the mmu_lock critical section.
> +        */
> +       kvm->mmu_invalidate_in_progress++;
> +
> +       if (likely(kvm->mmu_invalidate_in_progress == 1)) {
> +               kvm->mmu_invalidate_range_start = INVALID_GPA;
> +               kvm->mmu_invalidate_range_end = INVALID_GPA;
> +       }
> +}
> +
> +void kvm_mmu_invalidate_range_add(struct kvm *kvm, gfn_t start, gfn_t end)
> +{
> +       WARN_ON_ONCE(!kvm->mmu_invalidate_in_progress);
> +
> +       if (likely(kvm->mmu_invalidate_in_progress == 1)) {
> +               kvm->mmu_invalidate_range_start = start;
> +               kvm->mmu_invalidate_range_end = end;
> +       } else {
> +               /*
> +                * Fully tracking multiple concurrent ranges has diminishing
> +                * returns. Keep things simple and just find the minimal range
> +                * which includes the current and new ranges. As there won't be
> +                * enough information to subtract a range after its invalidate
> +                * completes, any ranges invalidated concurrently will
> +                * accumulate and persist until all outstanding invalidates
> +                * complete.
> +                */
> +               kvm->mmu_invalidate_range_start =
> +                       min(kvm->mmu_invalidate_range_start, start);
> +               kvm->mmu_invalidate_range_end =
> +                       max(kvm->mmu_invalidate_range_end, end);
> +       }
> +}
> +
> +void kvm_mmu_invalidate_end(struct kvm *kvm)
> +{
> +       /*
> +        * This sequence increase will notify the kvm page fault that
> +        * the page that is going to be mapped in the spte could have
> +        * been freed.
> +        */
> +       kvm->mmu_invalidate_seq++;
> +       smp_wmb();
> +       /*
> +        * The above sequence increase must be visible before the
> +        * below count decrease, which is ensured by the smp_wmb above
> +        * in conjunction with the smp_rmb in mmu_invalidate_retry().
> +        */
> +       kvm->mmu_invalidate_in_progress--;
> +}
> +
>  #if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
>  static inline struct kvm *mmu_notifier_to_kvm(struct mmu_notifier *mn)
>  {
> @@ -714,45 +770,6 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
>         kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
>  }
>
> -void kvm_mmu_invalidate_begin(struct kvm *kvm)
> -{
> -       /*
> -        * The count increase must become visible at unlock time as no
> -        * spte can be established without taking the mmu_lock and
> -        * count is also read inside the mmu_lock critical section.
> -        */
> -       kvm->mmu_invalidate_in_progress++;
> -
> -       if (likely(kvm->mmu_invalidate_in_progress == 1)) {
> -               kvm->mmu_invalidate_range_start = INVALID_GPA;
> -               kvm->mmu_invalidate_range_end = INVALID_GPA;
> -       }
> -}
> -
> -void kvm_mmu_invalidate_range_add(struct kvm *kvm, gfn_t start, gfn_t end)
> -{
> -       WARN_ON_ONCE(!kvm->mmu_invalidate_in_progress);
> -
> -       if (likely(kvm->mmu_invalidate_in_progress == 1)) {
> -               kvm->mmu_invalidate_range_start = start;
> -               kvm->mmu_invalidate_range_end = end;
> -       } else {
> -               /*
> -                * Fully tracking multiple concurrent ranges has diminishing
> -                * returns. Keep things simple and just find the minimal range
> -                * which includes the current and new ranges. As there won't be
> -                * enough information to subtract a range after its invalidate
> -                * completes, any ranges invalidated concurrently will
> -                * accumulate and persist until all outstanding invalidates
> -                * complete.
> -                */
> -               kvm->mmu_invalidate_range_start =
> -                       min(kvm->mmu_invalidate_range_start, start);
> -               kvm->mmu_invalidate_range_end =
> -                       max(kvm->mmu_invalidate_range_end, end);
> -       }
> -}
> -
>  static bool kvm_mmu_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>         kvm_mmu_invalidate_range_add(kvm, range->start, range->end);
> @@ -806,23 +823,6 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>         return 0;
>  }
>
> -void kvm_mmu_invalidate_end(struct kvm *kvm)
> -{
> -       /*
> -        * This sequence increase will notify the kvm page fault that
> -        * the page that is going to be mapped in the spte could have
> -        * been freed.
> -        */
> -       kvm->mmu_invalidate_seq++;
> -       smp_wmb();
> -       /*
> -        * The above sequence increase must be visible before the
> -        * below count decrease, which is ensured by the smp_wmb above
> -        * in conjunction with the smp_rmb in mmu_invalidate_retry().
> -        */
> -       kvm->mmu_invalidate_in_progress--;
> -}
> -
>  static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
>                                         const struct mmu_notifier_range *range)
>  {
> @@ -1140,6 +1140,11 @@ int __weak kvm_arch_create_vm_debugfs(struct kvm *kvm)
>         return 0;
>  }
>
> +bool __weak kvm_arch_has_private_mem(struct kvm *kvm)
> +{
> +       return false;
> +}
> +
>  static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
>  {
>         struct kvm *kvm = kvm_arch_alloc_vm();
> @@ -2349,15 +2354,47 @@ static u64 kvm_supported_mem_attributes(struct kvm *kvm)
>         return 0;
>  }
>
> +static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end)
> +{
> +       struct kvm_gfn_range gfn_range;
> +       struct kvm_memory_slot *slot;
> +       struct kvm_memslots *slots;
> +       struct kvm_memslot_iter iter;
> +       int i;
> +       int r = 0;
> +
> +       gfn_range.pte = __pte(0);
> +       gfn_range.may_block = true;
> +
> +       for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> +               slots = __kvm_memslots(kvm, i);
> +
> +               kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
> +                       slot = iter.slot;
> +                       gfn_range.start = max(start, slot->base_gfn);
> +                       gfn_range.end = min(end, slot->base_gfn + slot->npages);
> +                       if (gfn_range.start >= gfn_range.end)
> +                               continue;
> +                       gfn_range.slot = slot;
> +
> +                       r |= kvm_unmap_gfn_range(kvm, &gfn_range);
> +               }
> +       }
> +
> +       if (r)
> +               kvm_flush_remote_tlbs(kvm);
> +}
> +
>  static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
>                                            struct kvm_memory_attributes *attrs)
>  {
>         gfn_t start, end;
>         unsigned long i;
>         void *entry;
> +       int idx;
>         u64 supported_attrs = kvm_supported_mem_attributes(kvm);
>
> -       /* flags is currently not used. */
> +       /* 'flags' is currently not used. */
>         if (attrs->flags)
>                 return -EINVAL;
>         if (attrs->attributes & ~supported_attrs)
> @@ -2372,6 +2409,13 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
>
>         entry = attrs->attributes ? xa_mk_value(attrs->attributes) : NULL;
>
> +       if (kvm_arch_has_private_mem(kvm)) {
> +               KVM_MMU_LOCK(kvm);
> +               kvm_mmu_invalidate_begin(kvm);
> +               kvm_mmu_invalidate_range_add(kvm, start, end);
> +               KVM_MMU_UNLOCK(kvm);
> +       }
> +
>         mutex_lock(&kvm->lock);
>         for (i = start; i < end; i++)
>                 if (xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> @@ -2379,6 +2423,16 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
>                         break;
>         mutex_unlock(&kvm->lock);
>
> +       if (kvm_arch_has_private_mem(kvm)) {
> +               idx = srcu_read_lock(&kvm->srcu);
> +               KVM_MMU_LOCK(kvm);
> +               if (i > start)
> +                       kvm_unmap_mem_range(kvm, start, i);
> +               kvm_mmu_invalidate_end(kvm);
> +               KVM_MMU_UNLOCK(kvm);
> +               srcu_read_unlock(&kvm->srcu, idx);
> +       }
> +
>         attrs->address = i << PAGE_SHIFT;
>         attrs->size = (end - i) << PAGE_SHIFT;
>
> --
> 2.25.1
>