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

[Fuad Tabba]

Hi,

On Tue, Dec 6, 2022 at 12:01 PM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > > ---
> > >  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.

That was just a nit, please feel free to ignore it, especially if it
might cause headaches in the future with merges.
>
> >
> > >          */
> > > -       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.

With this fixed,

Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>
And the necessary work to port to arm64 (on qemu/arm64):
Tested-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad


>
> 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
> > >