Re: [PATCH v10 5/9] KVM: Use gfn instead of hva for mmu_notifier_retry

[Chao Peng]

On Mon, Dec 05, 2022 at 09:23:49AM +0000, Fuad Tabba wrote:
> Hi Chao,
> 
> On Fri, Dec 2, 2022 at 6:19 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> >
> > Currently in mmu_notifier invalidate path, hva range is recorded and
> > then checked against by mmu_notifier_retry_hva() in the page fault
> > handling 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 hva based shared memory, gfn is expected to also 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.
> >
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c   |  8 +++++---
> >  include/linux/kvm_host.h | 33 +++++++++++++++++++++------------
> >  virt/kvm/kvm_main.c      | 32 +++++++++++++++++++++++---------
> >  3 files changed, 49 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 4736d7849c60..e2c70b5afa3e 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4259,7 +4259,7 @@ 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)
> > @@ -6098,7 +6098,9 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t 
> > gfn_start, gfn_t gfn_end)
> >
> >         write_lock(&kvm->mmu_lock);
> >
> > -       kvm_mmu_invalidate_begin(kvm, gfn_start, gfn_end);
> > +       kvm_mmu_invalidate_begin(kvm);
> > +
> > +       kvm_mmu_invalidate_range_add(kvm, gfn_start, gfn_end);
> >
> >         flush = kvm_rmap_zap_gfn_range(kvm, gfn_start, gfn_end);
> >
> > @@ -6112,7 +6114,7 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t 
> > gfn_start, gfn_t gfn_end)
> >                 kvm_flush_remote_tlbs_with_address(kvm, gfn_start,
> >                                                    gfn_end - gfn_start);
> >
> > -       kvm_mmu_invalidate_end(kvm, gfn_start, gfn_end);
> > +       kvm_mmu_invalidate_end(kvm);
> >
> >         write_unlock(&kvm->mmu_lock);
> >  }
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 02347e386ea2..3d69484d2704 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -787,8 +787,8 @@ 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;
> > @@ -1389,10 +1389,9 @@ 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);
> > +void kvm_mmu_invalidate_range_add(struct kvm *kvm, gfn_t start, gfn_t end);
> > +void kvm_mmu_invalidate_end(struct kvm *kvm);
> >
> >  long kvm_arch_dev_ioctl(struct file *filp,
> >                         unsigned int ioctl, unsigned long arg);
> > @@ -1963,9 +1962,9 @@ 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);
> >         /*
> > @@ -1974,10 +1973,20 @@ static inline int mmu_invalidate_retry_hva(struct 
> > kvm *kvm,
> >          * that might be being invalidated. Note that it may include some 
> > false
> 
> nit: "might be" (or) "is being"
> 
> >          * positives, due to shortcuts when handing concurrent 
> > invalidations.
> 
> nit: handling

Both are existing code, but I can fix it either.

> 
> >          */
> > -       if (unlikely(kvm->mmu_invalidate_in_progress) &&
> > -           hva >= kvm->mmu_invalidate_range_start &&
> > -           hva < kvm->mmu_invalidate_range_end)
> > -               return 1;
> > +       if (unlikely(kvm->mmu_invalidate_in_progress)) {
> > +               /*
> > +                * Dropping mmu_lock after bumping 
> > mmu_invalidate_in_progress
> > +                * but before updating the range is a KVM bug.
> > +                */
> > +               if (WARN_ON_ONCE(kvm->mmu_invalidate_range_start == 
> > INVALID_GPA ||
> > +                                kvm->mmu_invalidate_range_end == 
> > INVALID_GPA))
> 
> INVALID_GPA is an x86-specific define in
> arch/x86/include/asm/kvm_host.h, so this doesn't build on other
> architectures. The obvious fix is to move it to
> include/linux/kvm_host.h.

Hmm, INVALID_GPA is defined as ZERO for x86, not 100% confident this is
correct choice for other architectures, but after search it has not been
used for other architectures, so should be safe to make it common.

Thanks,
Chao
> 
> Cheers,
> /fuad
> 
> > +                       return 1;
> > +
> > +               if (gfn >= kvm->mmu_invalidate_range_start &&
> > +                   gfn < kvm->mmu_invalidate_range_end)
> > +                       return 1;
> > +       }
> > +
> >         if (kvm->mmu_invalidate_seq != mmu_seq)
> >                 return 1;
> >         return 0;
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index b882eb2c76a2..ad55dfbc75d7 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -540,9 +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);
> >  typedef void (*on_unlock_fn_t)(struct kvm *kvm);
> >
> >  struct kvm_hva_range {
> > @@ -628,7 +626,8 @@ 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);
> > +
> >                                 if (IS_KVM_NULL_FN(range->handler))
> >                                         break;
> >                         }
> > @@ -715,8 +714,7 @@ 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)
> > +void kvm_mmu_invalidate_begin(struct kvm *kvm)
> >  {
> >         /*
> >          * The count increase must become visible at unlock time as no
> > @@ -724,6 +722,17 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, 
> > unsigned long start,
> >          * 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;
> > @@ -744,6 +753,12 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, 
> > unsigned long start,
> >         }
> >  }
> >
> > +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);
> > +       return kvm_unmap_gfn_range(kvm, range);
> > +}
> > +
> >  static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> >                                         const struct mmu_notifier_range 
> > *range)
> >  {
> > @@ -752,7 +767,7 @@ 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,
> > +               .handler        = kvm_mmu_unmap_gfn_range,
> >                 .on_lock        = kvm_mmu_invalidate_begin,
> >                 .on_unlock      = kvm_arch_guest_memory_reclaimed,
> >                 .flush_on_ret   = true,
> > @@ -791,8 +806,7 @@ 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)
> >  {
> >         /*
> >          * This sequence increase will notify the kvm page fault that
> > --
> > 2.25.1
> >