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

[Chao Peng]

On Wed, Dec 07, 2022 at 04:13:14PM +0800, Yuan Yao wrote:
> On Fri, Dec 02, 2022 at 02:13:44PM +0800, Chao Peng 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().
> >
> > 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@xxxxxxxxxxxxxxx>
> > ---
> >  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);
> 
> Nit: this works for KVM_MEMORY_ATTRIBUTE_PRIVATE, but
> the invalidation should be necessary yet for attribute change of:
> 
> KVM_MEMORY_ATTRIBUTE_READ
> KVM_MEMORY_ATTRIBUTE_WRITE
> KVM_MEMORY_ATTRIBUTE_EXECUTE

The unmapping is only needed for confidential usages which uses
KVM_MEMORY_ATTRIBUTE_PRIVATE only and the other flags are defined here
for other usages like pKVM. As Fuad commented in a different reply, pKVM
supports in-place remapping and unmapping is unnecessary.

Thanks,
Chao
> 
> > +		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);
> 
> Ditto.
> 
> > +		KVM_MMU_UNLOCK(kvm);
> > +		srcu_read_unlock(&kvm->srcu, idx);
> > +	}
> > +
> >  	attrs->address = i << PAGE_SHIFT;
> >  	attrs->size = (end - i) << PAGE_SHIFT;
> >
> > --
> > 2.25.1
> >
> >