Subject: Re: [PATCH v3 3/5] KVM: arm64: nv: Avoid full shadow s2 unmap

From: Marc Zyngier <[email-masked]>
Date: Thu, 28 May 2026 13:59:11 +0100

On Sun, 10 May 2026 15:53:36 +0100,
Wei-Lin Chang <[email-masked]> wrote:
> 
> Currently we are forced to fully unmap all shadow stage-2 for a VM when
> unmapping a page from the canonical stage-2, for example during an MMU
> notifier call. This is because we are not tracking what canonical IPA
> are mapped in the shadow stage-2 page tables hence there is no way to
> know what to unmap.
> 
> Create a per kvm_s2_mmu maple tree to track canonical IPA range ->
> nested IPA range, so that it is possible to partially unmap shadow
> stage-2 when a canonical IPA range is unmapped. The algorithm is simple
> and conservative:
> 
> At each shadow stage-2 map, insert the nested IPA range into the maple
> tree, with the canonical IPA range as the key. If the canonical IPA
> range doesn't overlap with existing ranges in the tree, insert as is,
> and a reverse mapping for this range is established. But if the
> canonical IPA range overlaps with any existing ranges in the tree,
> create a new range that spans all the overlapping ranges including the
> input range and replace those existing ranges. In the mean time, mark
> this new spanning canonical IPA range with an "UNKNOWN_IPA" bit,
> indicating we give up tracking the nested IPA ranges that map to this
> canonical IPA range.
> 
> The maple tree's 64 bit entry is enough to store the nested IPA and
> the UNKNOWN_IPA status, therefore besides maple tree's internal
> operation, memory allocation is avoided.
> 
> Example:
> |||| means existing range, ---- means empty range
> 
> input:            $$$$$$$$$$$$$$$$$$$$$$$$$$
> tree:  --||||-----|||||||---------||||||||||-----------
> 
> insert spanning range and replace overlapping ones:
>        --||||-----||||||||||||||||||||||||||-----------
>                   ^^^^marked UNKNOWN_IPA^^^^
> 
> With the reverse map created, when a canonical IPA range gets unmapped,
> look into each s2 mmu's maple tree and look for canonical IPA ranges
> affected, and base on their UNKNOWN_IPA status:
> 
> UNKNOWN_IPA     -> fall back and fully unmap the current shadow
>                    stage-2, also clear the tree
> 
> not UNKNOWN_IPA -> unmap the nested IPA range, and remove the reverse
>                    map entry
> 
> Suggested-by: Marc Zyngier <[email-masked]>
> Signed-off-by: Wei-Lin Chang <[email-masked]>
> ---
>  arch/arm64/include/asm/kvm_host.h   |   4 +
>  arch/arm64/include/asm/kvm_nested.h |   4 +
>  arch/arm64/kvm/mmu.c                |  27 ++++--
>  arch/arm64/kvm/nested.c             | 140 +++++++++++++++++++++++++++-
>  4 files changed, 167 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 1a56d137df10..dc4c0bce1bbb 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -223,6 +223,10 @@ struct kvm_s2_mmu {
>  	 */
>  	bool	pending_unmap;
>  
> +	bool	nested_revmap_broken;
> +	/* canonical IPA to nested IPA range lookup */
> +	struct maple_tree nested_revmap_mt;
> +
>  #ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
>  	struct dentry *shadow_pt_debugfs_dentry;
>  #endif
> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> index 091544e6af44..5cbf78dfc685 100644
> --- a/arch/arm64/include/asm/kvm_nested.h
> +++ b/arch/arm64/include/asm/kvm_nested.h
> @@ -76,6 +76,8 @@ extern void kvm_s2_mmu_iterate_by_vmid(struct kvm *kvm, u16 vmid,
>  				       const union tlbi_info *info,
>  				       void (*)(struct kvm_s2_mmu *,
>  						const union tlbi_info *));
> +extern void kvm_record_nested_revmap(gpa_t gpa, struct kvm_s2_mmu *mmu,
> +				     gpa_t fault_ipa, size_t map_size);
>  extern void kvm_vcpu_load_hw_mmu(struct kvm_vcpu *vcpu);
>  extern void kvm_vcpu_put_hw_mmu(struct kvm_vcpu *vcpu);
>  
> @@ -164,6 +166,8 @@ extern int kvm_s2_handle_perm_fault(struct kvm_vcpu *vcpu,
>  				    struct kvm_s2_trans *trans);
>  extern int kvm_inject_s2_fault(struct kvm_vcpu *vcpu, u64 esr_el2);
>  extern void kvm_nested_s2_wp(struct kvm *kvm);
> +extern void kvm_unmap_gfn_range_nested(struct kvm *kvm, gpa_t gpa, size_t size,
> +				       bool may_block);
>  extern void kvm_nested_s2_unmap(struct kvm *kvm, bool may_block);
>  extern void kvm_nested_s2_flush(struct kvm *kvm);
>  
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index e4becd5cdf36..ce0bd88cd3c1 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -5,6 +5,7 @@
>   */
>  
>  #include <linux/acpi.h>
> +#include <linux/maple_tree.h>
>  #include <linux/mman.h>
>  #include <linux/kvm_host.h>
>  #include <linux/io.h>
> @@ -1099,6 +1100,7 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>  {
>  	struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu);
>  	struct kvm_pgtable *pgt = NULL;
> +	struct maple_tree *revmap_mt = &mmu->nested_revmap_mt;
>  
>  	write_lock(&kvm->mmu_lock);
>  	pgt = mmu->pgt;
> @@ -1108,8 +1110,11 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>  		free_percpu(mmu->last_vcpu_ran);
>  	}
>  
> -	if (kvm_is_nested_s2_mmu(kvm, mmu))
> +	if (kvm_is_nested_s2_mmu(kvm, mmu)) {
> +		if (!mtree_empty(revmap_mt))
> +			mtree_destroy(revmap_mt);
>  		kvm_init_nested_s2_mmu(mmu);
> +	}
>  
>  	write_unlock(&kvm->mmu_lock);
>  
> @@ -1631,6 +1636,10 @@ static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
>  		goto out_unlock;
>  	}
>  
> +	if (s2fd->nested)
> +		kvm_record_nested_revmap(gfn << PAGE_SHIFT, pgt->mmu,
> +					 s2fd->fault_ipa, PAGE_SIZE);
> +
>  	ret = KVM_PGT_FN(kvm_pgtable_stage2_map)(pgt, s2fd->fault_ipa, PAGE_SIZE,
>  						 __pfn_to_phys(pfn), prot,
>  						 memcache, flags);
> @@ -2034,6 +2043,10 @@ static int kvm_s2_fault_map(const struct kvm_s2_fault_desc *s2fd,
>  		ret = KVM_PGT_FN(kvm_pgtable_stage2_relax_perms)(pgt, gfn_to_gpa(gfn),
>  								 prot, flags);
>  	} else {
> +		if (s2fd->nested)
> +			kvm_record_nested_revmap(canonical_gpa, pgt->mmu,
> +						 gfn_to_gpa(gfn), mapping_size);
> +
>  		ret = KVM_PGT_FN(kvm_pgtable_stage2_map)(pgt, gfn_to_gpa(gfn), mapping_size,
>  							 __pfn_to_phys(pfn), prot,
>  							 memcache, flags);
> @@ -2389,14 +2402,16 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
>  
>  bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
> +	gpa_t gpa = range->start << PAGE_SHIFT;
> +	size_t size = (range->end - range->start) << PAGE_SHIFT;
> +	bool may_block = range->may_block;
> +
>  	if (!kvm->arch.mmu.pgt || kvm_vm_is_protected(kvm))
>  		return false;
>  
> -	__unmap_stage2_range(&kvm->arch.mmu, range->start << PAGE_SHIFT,
> -			     (range->end - range->start) << PAGE_SHIFT,
> -			     range->may_block);
> +	__unmap_stage2_range(&kvm->arch.mmu, gpa, size, may_block);

This sort of cleanups could be in a separate patch.

> +	kvm_unmap_gfn_range_nested(kvm, gpa, size, may_block);
>  
> -	kvm_nested_s2_unmap(kvm, range->may_block);
>  	return false;
>  }
>  
> @@ -2674,7 +2689,7 @@ void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
>  
>  	write_lock(&kvm->mmu_lock);
>  	kvm_stage2_unmap_range(&kvm->arch.mmu, gpa, size, true);
> -	kvm_nested_s2_unmap(kvm, true);
> +	kvm_unmap_gfn_range_nested(kvm, gpa, size, true);
>  	write_unlock(&kvm->mmu_lock);
>  }
>  
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index 883b6c1008fb..35b5d5f21a23 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -7,6 +7,7 @@
>  #include <linux/bitfield.h>
>  #include <linux/kvm.h>
>  #include <linux/kvm_host.h>
> +#include <linux/maple_tree.h>
>  
>  #include <asm/fixmap.h>
>  #include <asm/kvm_arm.h>
> @@ -43,6 +44,20 @@ struct vncr_tlb {
>   */
>  #define S2_MMU_PER_VCPU		2
>  
> +/*
> + * Per shadow S2 reverse map (IPA -> nested IPA range) maple tree payload
> + * layout:
> + *
> + * bit  62:     valid, prevents the case where the nested IPA is 0 and turning
> + *              the whole value to 0
> + * bits 55-12:  nested IPA bits 55-12
> + * bit  0:      UNKNOWN_IPA bit, 1 indicates we give up on tracking what nested
> + *              IPA maps to this canonical IPA in the shadow stage-2
> + */
> +#define VALID_ENTRY		BIT(62)
> +#define ADDR_MASK		GENMASK_ULL(55, 12)
> +#define UNKNOWN_IPA		BIT(0)
> +
>  void kvm_init_nested(struct kvm *kvm)
>  {
>  	kvm->arch.nested_mmus = NULL;
> @@ -769,12 +784,57 @@ static struct kvm_s2_mmu *get_s2_mmu_nested(struct kvm_vcpu *vcpu)
>  	return s2_mmu;
>  }
>  
> +void kvm_record_nested_revmap(gpa_t ipa, struct kvm_s2_mmu *mmu,
> +			      gpa_t fault_ipa, size_t map_size)

The name fault_ipa doesn't really make sense here. This is the IPA as
seen from L1 (the input to the L1 S2 tables). We indeed obtain it from
a fault, but that should not influence the naming here.

Similarly, 'ipa' should be qualified a bit better to reflect that this
is the L0 IPA.

> +{
> +	struct maple_tree *revmap_mt = &mmu->nested_revmap_mt;
> +	gpa_t ipa_end = ipa + map_size - 1;

Are you always guaranteed that ipa is aligned on map_size?

> +	u64 entry, new_entry = 0;
> +	MA_STATE(mas_rev, revmap_mt, ipa, ipa_end);
> +
> +	if (mmu->nested_revmap_broken)
> +		return;
> +
> +	mtree_lock(revmap_mt);
> +	entry = xa_to_value(mas_find_range(&mas_rev, ipa_end));
> +
> +	if (entry) {
> +		/* maybe just a perm update... */
> +		if (!(entry & UNKNOWN_IPA) && mas_rev.index == ipa &&

Shouldn't you check that VALID_ENTRY is set? Is the index guaranteed
to match the L0 IPA?

> +		    mas_rev.last == ipa_end &&
> +		    fault_ipa == (entry & ADDR_MASK))

Again, I think there is a potential alignment issue here.

> +			goto unlock;
> +		/*
> +		 * Create a "UNKNOWN_IPA" range that spans all the overlapping
> +		 * ranges and store it.
> +		 */
> +		while (entry && mas_rev.index <= ipa_end) {
> +			ipa = min(mas_rev.index, ipa);
> +			ipa_end = max(mas_rev.last, ipa_end);
> +			entry = xa_to_value(mas_find_range(&mas_rev, ipa_end));
> +		}
> +		new_entry |= UNKNOWN_IPA;
> +	} else {
> +		new_entry |= fault_ipa;
> +		new_entry |= VALID_ENTRY;
> +	}
> +
> +	mas_set_range(&mas_rev, ipa, ipa_end);
> +	if (mas_store_gfp(&mas_rev, xa_mk_value(new_entry),
> +			  GFP_NOWAIT | __GFP_ACCOUNT))
> +		mmu->nested_revmap_broken = true;

I really think we ought to track this event happening. Maybe a trace
point.

> +unlock:
> +	mtree_unlock(revmap_mt);
> +}
> +
>  void kvm_init_nested_s2_mmu(struct kvm_s2_mmu *mmu)
>  {
>  	/* CnP being set denotes an invalid entry */
>  	mmu->tlb_vttbr = VTTBR_CNP_BIT;
>  	mmu->nested_stage2_enabled = false;
>  	atomic_set(&mmu->refcnt, 0);
> +	mt_init(&mmu->nested_revmap_mt);
> +	mmu->nested_revmap_broken = false;
>  }
>  
>  void kvm_vcpu_load_hw_mmu(struct kvm_vcpu *vcpu)
> @@ -1150,6 +1210,82 @@ void kvm_nested_s2_wp(struct kvm *kvm)
>  	kvm_invalidate_vncr_ipa(kvm, 0, BIT(kvm->arch.mmu.pgt->ia_bits));
>  }
>  
> +static void reset_revmap_and_unmap(struct kvm_s2_mmu *mmu, bool may_block)
> +{
> +	mtree_destroy(&mmu->nested_revmap_mt);
> +	mmu->nested_revmap_broken = false;
> +	kvm_stage2_unmap_range(mmu, 0, kvm_phys_size(mmu), may_block);
> +}
> +
> +static void unmap_mmu_ipa_range(struct kvm_s2_mmu *mmu, gpa_t gpa,
> +				  size_t unmap_size, bool may_block)

Same comment as above about the nature of 'gpa'. I *think* this is the
L0 IPA, but please clarify.

> +{
> +	struct maple_tree *revmap_mt = &mmu->nested_revmap_mt;
> +	gpa_t ipa = gpa;
> +	gpa_t ipa_end = gpa + unmap_size - 1;

Similar concerns about alignments.

> +	u64 entry;
> +	size_t entry_size;
> +	MA_STATE(mas_rev, revmap_mt, gpa, ipa_end);
> +
> +	if (mmu->nested_revmap_broken) {
> +		reset_revmap_and_unmap(mmu, may_block);
> +		return;
> +	}
> +
> +	mtree_lock(revmap_mt);
> +	entry = xa_to_value(mas_find_range(&mas_rev, ipa_end));
> +
> +	while (entry && mas_rev.index <= ipa_end) {

I'm again concerned that the VALID bit is never checked.

> +		ipa = mas_rev.last + 1;
> +		entry_size = mas_rev.last - mas_rev.index + 1;
> +		/*
> +		 * Give up and invalidate this s2 mmu if the unmap range
> +		 * touches any UNKNOWN_IPA range.
> +		 */
> +		if (entry & UNKNOWN_IPA) {
> +			mtree_unlock(revmap_mt);
> +			reset_revmap_and_unmap(mmu, may_block);
> +			return;
> +		}
> +
> +		/*
> +		 * Ignore result, it is okay if a reverse mapping erase
> +		 * fails.
> +		 */
> +		mas_store_gfp(&mas_rev, NULL, GFP_NOWAIT | __GFP_ACCOUNT);
> +
> +		mtree_unlock(revmap_mt);
> +		kvm_stage2_unmap_range(mmu, entry & ADDR_MASK, entry_size,
> +				       may_block);
> +		mtree_lock(revmap_mt);
> +		/*
> +		 * Other maple tree operations during preemption could render
> +		 * this ma_state invalid, so reset it.
> +		 */
> +		mas_set_range(&mas_rev, ipa, ipa_end);
> +		entry = xa_to_value(mas_find_range(&mas_rev, ipa_end));
> +	}
> +	mtree_unlock(revmap_mt);
> +}
> +
> +void kvm_unmap_gfn_range_nested(struct kvm *kvm, gpa_t gpa, size_t size,
> +				bool may_block)
> +{
> +	int i;
> +
> +	if (!kvm->arch.nested_mmus_size)
> +		return;
> +
> +	for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
> +		struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
> +
> +		if (kvm_s2_mmu_valid(mmu))
> +			unmap_mmu_ipa_range(mmu, gpa, size, may_block);
> +	}
> +
> +	kvm_invalidate_vncr_ipa(kvm, gpa, gpa + size);

I'm not overly fond of propagating the VNCR invalidation in all the S2
manipulations. I understand why you are doing it here, but I think we
need to have a better solution.

Fundamentally, VNCR invalidation has nothing to do with S2. This
really is a EL2 S1 thing. And given that you have a reverse map per
s2_mmu, it would be easy enough to track VNCR TLBs through that.

It doesn't have to be part of this patch, but that would be a good
thing to disentangle as a subsequent patch.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.