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

[Chao Peng <chao.p.peng@linux.intel.com>]

On Thu, Oct 27, 2022 at 11:29:14AM +0100, Fuad Tabba wrote:
> Hi,
> 
> On Tue, Oct 25, 2022 at 4:19 PM Chao Peng <chao.p.peng@linux.intel.com> 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().

Forgot to mention: the bug is still with kvm_zap_gfn_range(). It calls
kvm_mmu_invalidate_begin/end with a gfn range but before this series
kvm_mmu_invalidate_begin/end actually accept a hva range. Note it's
unrelated to whether we use kvm_zap_gfn_range() or kvm_unmap_gfn_range()
in the following patch (patch 05).

Thanks,
Chao
> 
> > gfn when calling kvm_mmu_invalidate_begin/end() while these functions
> > accept hva in current code.
> >
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> 
> Based on reading this code and my limited knowledge of the x86 MMU code:
> Reviewed-by: Fuad Tabba <tabba@google.com>
> 
> 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
> >