Re: [PATCH v9 4/8] KVM: Use gfn instead of hva for mmu_notifier_retry

[Fuad Tabba]

Hi,

On Tue, Oct 25, 2022 at 4:19 PM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
>
> Currently in mmu_notifier validate path, hva range is recorded and then
> checked against in the mmu_notifier_retry_hva() of the page fault path.
> However, for the to be introduced private memory, a page fault may not
> have a hva associated, checking gfn(gpa) makes more sense.
>
> For existing non private memory case, gfn is expected to continue to
> work. The only downside is when aliasing multiple gfns to a single hva,
> the current algorithm of checking multiple ranges could result in a much
> larger range being rejected. Such aliasing should be uncommon, so the
> impact is expected small.
>
> It also fixes a bug in kvm_zap_gfn_range() which has already been using

nit: Now it's kvm_unmap_gfn_range().

> gfn when calling kvm_mmu_invalidate_begin/end() while these functions
> accept hva in current code.
>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---

Based on reading this code and my limited knowledge of the x86 MMU code:
Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad


>  arch/x86/kvm/mmu/mmu.c   |  2 +-
>  include/linux/kvm_host.h | 18 +++++++---------
>  virt/kvm/kvm_main.c      | 45 ++++++++++++++++++++++++++--------------
>  3 files changed, 39 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6f81539061d6..33b1aec44fb8 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4217,7 +4217,7 @@ static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
>                 return true;
>
>         return fault->slot &&
> -              mmu_invalidate_retry_hva(vcpu->kvm, mmu_seq, fault->hva);
> +              mmu_invalidate_retry_gfn(vcpu->kvm, mmu_seq, fault->gfn);
>  }
>
>  static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 739a7562a1f3..79e5cbc35fcf 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -775,8 +775,8 @@ struct kvm {
>         struct mmu_notifier mmu_notifier;
>         unsigned long mmu_invalidate_seq;
>         long mmu_invalidate_in_progress;
> -       unsigned long mmu_invalidate_range_start;
> -       unsigned long mmu_invalidate_range_end;
> +       gfn_t mmu_invalidate_range_start;
> +       gfn_t mmu_invalidate_range_end;
>  #endif
>         struct list_head devices;
>         u64 manual_dirty_log_protect;
> @@ -1365,10 +1365,8 @@ void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc);
>  void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
>  #endif
>
> -void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> -                             unsigned long end);
> -void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
> -                           unsigned long end);
> +void kvm_mmu_invalidate_begin(struct kvm *kvm, gfn_t start, gfn_t end);
> +void kvm_mmu_invalidate_end(struct kvm *kvm, gfn_t start, gfn_t end);
>
>  long kvm_arch_dev_ioctl(struct file *filp,
>                         unsigned int ioctl, unsigned long arg);
> @@ -1937,9 +1935,9 @@ static inline int mmu_invalidate_retry(struct kvm *kvm, unsigned long mmu_seq)
>         return 0;
>  }
>
> -static inline int mmu_invalidate_retry_hva(struct kvm *kvm,
> +static inline int mmu_invalidate_retry_gfn(struct kvm *kvm,
>                                            unsigned long mmu_seq,
> -                                          unsigned long hva)
> +                                          gfn_t gfn)
>  {
>         lockdep_assert_held(&kvm->mmu_lock);
>         /*
> @@ -1949,8 +1947,8 @@ static inline int mmu_invalidate_retry_hva(struct kvm *kvm,
>          * positives, due to shortcuts when handing concurrent invalidations.
>          */
>         if (unlikely(kvm->mmu_invalidate_in_progress) &&
> -           hva >= kvm->mmu_invalidate_range_start &&
> -           hva < kvm->mmu_invalidate_range_end)
> +           gfn >= kvm->mmu_invalidate_range_start &&
> +           gfn < kvm->mmu_invalidate_range_end)
>                 return 1;
>         if (kvm->mmu_invalidate_seq != mmu_seq)
>                 return 1;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 8dace78a0278..09c9cdeb773c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -540,8 +540,7 @@ static void kvm_mmu_notifier_invalidate_range(struct mmu_notifier *mn,
>
>  typedef bool (*hva_handler_t)(struct kvm *kvm, struct kvm_gfn_range *range);
>
> -typedef void (*on_lock_fn_t)(struct kvm *kvm, unsigned long start,
> -                            unsigned long end);
> +typedef void (*on_lock_fn_t)(struct kvm *kvm, gfn_t start, gfn_t end);
>
>  typedef void (*on_unlock_fn_t)(struct kvm *kvm);
>
> @@ -628,7 +627,8 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
>                                 locked = true;
>                                 KVM_MMU_LOCK(kvm);
>                                 if (!IS_KVM_NULL_FN(range->on_lock))
> -                                       range->on_lock(kvm, range->start, range->end);
> +                                       range->on_lock(kvm, gfn_range.start,
> +                                                           gfn_range.end);
>                                 if (IS_KVM_NULL_FN(range->handler))
>                                         break;
>                         }
> @@ -715,15 +715,9 @@ 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, unsigned long start,
> -                             unsigned long end)
> +static inline void update_invalidate_range(struct kvm *kvm, gfn_t start,
> +                                                           gfn_t end)
>  {
> -       /*
> -        * 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 = start;
>                 kvm->mmu_invalidate_range_end = end;
> @@ -744,6 +738,28 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
>         }
>  }
>
> +static void mark_invalidate_in_progress(struct kvm *kvm, gfn_t start, gfn_t end)
> +{
> +       /*
> +        * 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++;
> +}
> +
> +static bool kvm_mmu_handle_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
> +{
> +       update_invalidate_range(kvm, range->start, range->end);
> +       return kvm_unmap_gfn_range(kvm, range);
> +}
> +
> +void kvm_mmu_invalidate_begin(struct kvm *kvm, gfn_t start, gfn_t end)
> +{
> +       mark_invalidate_in_progress(kvm, start, end);
> +       update_invalidate_range(kvm, start, end);
> +}
> +
>  static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>                                         const struct mmu_notifier_range *range)
>  {
> @@ -752,8 +768,8 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>                 .start          = range->start,
>                 .end            = range->end,
>                 .pte            = __pte(0),
> -               .handler        = kvm_unmap_gfn_range,
> -               .on_lock        = kvm_mmu_invalidate_begin,
> +               .handler        = kvm_mmu_handle_gfn_range,
> +               .on_lock        = mark_invalidate_in_progress,
>                 .on_unlock      = kvm_arch_guest_memory_reclaimed,
>                 .flush_on_ret   = true,
>                 .may_block      = mmu_notifier_range_blockable(range),
> @@ -791,8 +807,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>         return 0;
>  }
>
> -void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
> -                           unsigned long end)
> +void kvm_mmu_invalidate_end(struct kvm *kvm, gfn_t start, gfn_t end)
>  {
>         /*
>          * This sequence increase will notify the kvm page fault that
> --
> 2.25.1
>