Re: [PATCH v13 15/48] arm64: RMI: RTT tear down

[Wei-Lin Chang]

On Fri, Mar 20, 2026 at 04:12:48PM +0000, Steven Price wrote:
> On 19/03/2026 17:35, Wei-Lin Chang wrote:
> > On Wed, Mar 18, 2026 at 03:53:39PM +0000, Steven Price wrote:
> >> The RMM owns the stage 2 page tables for a realm, and KVM must request
> >> that the RMM creates/destroys entries as necessary. The physical pages
> >> to store the page tables are delegated to the realm as required, and can
> >> be undelegated when no longer used.
> >>
> >> Creating new RTTs is the easy part, tearing down is a little more
> >> tricky. The result of realm_rtt_destroy() can be used to effectively
> >> walk the tree and destroy the entries (undelegating pages that were
> >> given to the realm).
> >>
> >> Signed-off-by: Steven Price <steven.price@xxxxxxx>
> >> ---
> >> Changes since v12:
> >>  * Simplify some functions now we know RMM page size is the same as the
> >>    host's.
> >> Changes since v11:
> >>  * Moved some code from earlier in the series to this one so that it's
> >>    added when it's first used.
> >> Changes since v10:
> >>  * RME->RMI rename.
> >>  * Some code to handle freeing stage 2 PGD moved into this patch where
> >>    it belongs.
> >> Changes since v9:
> >>  * Add a comment clarifying that root level RTTs are not destroyed until
> >>    after the RD is destroyed.
> >> Changes since v8:
> >>  * Introduce free_rtt() wrapper which calls free_delegated_granule()
> >>    followed by kvm_account_pgtable_pages(). This makes it clear where an
> >>    RTT is being freed rather than just a delegated granule.
> >> Changes since v6:
> >>  * Move rme_rtt_level_mapsize() and supporting defines from kvm_rme.h
> >>    into rme.c as they are only used in that file.
> >> Changes since v5:
> >>  * Rename some RME_xxx defines to do with page sizes as RMM_xxx - they are
> >>    a property of the RMM specification not the RME architecture.
> >> Changes since v2:
> >>  * Moved {alloc,free}_delegated_page() and ensure_spare_page() to a
> >>    later patch when they are actually used.
> >>  * Some simplifications now rmi_xxx() functions allow NULL as an output
> >>    parameter.
> >>  * Improved comments and code layout.
> >> ---
> >>  arch/arm64/include/asm/kvm_rmi.h |   7 ++
> >>  arch/arm64/kvm/mmu.c             |  15 +++-
> >>  arch/arm64/kvm/rmi.c             | 145 +++++++++++++++++++++++++++++++
> >>  3 files changed, 166 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm64/include/asm/kvm_rmi.h b/arch/arm64/include/asm/kvm_rmi.h
> >> index 0ada525af18f..16a297f3091a 100644
> >> --- a/arch/arm64/include/asm/kvm_rmi.h
> >> +++ b/arch/arm64/include/asm/kvm_rmi.h
> >> @@ -68,5 +68,12 @@ u32 kvm_realm_ipa_limit(void);
> >>  
> >>  int kvm_init_realm_vm(struct kvm *kvm);
> >>  void kvm_destroy_realm(struct kvm *kvm);
> >> +void kvm_realm_destroy_rtts(struct kvm *kvm);
> >> +
> >> +static inline bool kvm_realm_is_private_address(struct realm *realm,
> >> +						unsigned long addr)
> >> +{
> >> +	return !(addr & BIT(realm->ia_bits - 1));
> >> +}
> >>  
> >>  #endif /* __ASM_KVM_RMI_H */
> >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> >> index 9dc242c3b9c8..41152abf55b2 100644
> >> --- a/arch/arm64/kvm/mmu.c
> >> +++ b/arch/arm64/kvm/mmu.c
> >> @@ -1098,10 +1098,23 @@ void stage2_unmap_vm(struct kvm *kvm)
> >>  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 kvm_pgtable *pgt;
> >>  
> >>  	write_lock(&kvm->mmu_lock);
> >>  	pgt = mmu->pgt;
> >> +	if (kvm_is_realm(kvm) &&
> >> +	    (kvm_realm_state(kvm) != REALM_STATE_DEAD &&
> >> +	     kvm_realm_state(kvm) != REALM_STATE_NONE)) {
> >> +		write_unlock(&kvm->mmu_lock);
> >> +		kvm_realm_destroy_rtts(kvm);
> >> +
> >> +		/*
> >> +		 * The PGD pages can be reclaimed only after the realm (RD) is
> >> +		 * destroyed. We call this again from kvm_destroy_realm() after
> >> +		 * the RD is destroyed.
> >> +		 */
> >> +		return;
> >> +	}
> > 
> > Hi,
> > 
> > I see that kvm_free_stage2_pgd() will be called twice:
> > 
> > kvm_destroy_vm()
> >   mmu_notifier_unregister()
> >     kvm_mmu_notifier_release()
> >       kvm_flush_shadow_all()
> >         kvm_arch_flush_shadow_all()
> >           kvm_uninit_stage2_mmu()
> >             kvm_free_stage2_pgd()
> >   kvm_arch_destroy_vm()
> >     kvm_destroy_realm()
> >       kvm_free_stage2_pgd()
> > 
> > At the first call the realm state is REALM_STATE_ACTIVE, at the second
> > it is REALM_STATE_DEAD. Reading the comment added to
> > kvm_free_stage2_pgd() here, does it mean this function is called twice
> > on purpose? If so do you think it's better to extract this and create
> > another function instead, then use kvm_is_realm() to choose which to
> > run? I think it is confusing to have this function run twice for a
> > realm.
> 
> So the issue here is that the RMM requires we do things in a different
> order to a normal VM. For a realm the PGD cannot be destroyed until the
> realm itself is destroyed - the RMM revent the host undelegating them.
> 
> So the first call cannot actually do the free - this is the
> REALM_STATE_ACTIVE case.
> 
> In kvm_destroy_realm() we tear down the actual realm and undelegate the
> granules. We then need to actually free the PGD - the "obvious" way of
> doing that is calling kvm_free_stage2_pgd() as that handles the KVM
> intricacies - e.g. updating the mmu object.
> 
> I'm not sure how to structure the code better without causing
> duplication - I don't want another copy of the cleanup from
> kvm_free_stage2_pgd() in a CCA specific file because it will most likely
> get out of sync with the normal VM case. There is a comment added
> explaining "we call this again" which I hoped would make it less confusing.
> 

Oh, I see, thanks for letting me know!

During this I found in the first call of kvm_free_stage2_pgd() it's doing
kvm_stage2_unmap_range() and kvm_realm_destroy_rtts(), but they are also
called in kvm_destroy_realm(), is that intentional?
If they can be called at kvm_destroy_realm() time, could we just not do
kvm_free_stage2_pgd() in kvm_uninit_stage2_mmu() for realms?
And if they should be called in kvm_free_stage2_pgd(), could we refactor
it to something like:
(just showing the idea, didn't try compiling or anything)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 7d7caab8f573..280d2bef8492 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1030,9 +1030,25 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
 	return err;
 }
 
+static void kvm_realm_uninit_stage2(struct kvm_s2_mmu *mmu)
+{
+	struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu);
+	struct realm *realm = &kvm->arch.realm;
+
+	WARN_ON(kvm_realm_state(kvm) != REALM_STATE_ACTIVE);
+	write_lock(&kvm->mmu_lock);
+	kvm_stage2_unmap_range(mmu, 0, BIT(realm->ia_bits - 1), true);
+	write_unlock(&kvm->mmu_lock);
+	kvm_realm_destroy_rtts(kvm);
+}
+
 void kvm_uninit_stage2_mmu(struct kvm *kvm)
 {
-	kvm_free_stage2_pgd(&kvm->arch.mmu);
+	if (kvm_is_realm(kvm))
+		kvm_realm_uninit_stage2(&kvm->arch.mmu);
+	else
+		kvm_free_stage2_pgd(&kvm->arch.mmu);
+
 	kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
 }
 
@@ -1117,22 +1133,7 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
 
 	write_lock(&kvm->mmu_lock);
 	pgt = mmu->pgt;
-	if (kvm_is_realm(kvm) &&
-	    (kvm_realm_state(kvm) != REALM_STATE_DEAD &&
-	     kvm_realm_state(kvm) != REALM_STATE_NONE)) {
-		struct realm *realm = &kvm->arch.realm;
-
-		kvm_stage2_unmap_range(mmu, 0, BIT(realm->ia_bits - 1), true);
-		write_unlock(&kvm->mmu_lock);
-		kvm_realm_destroy_rtts(kvm);
 
-		/*
-		 * The PGD pages can be reclaimed only after the realm (RD) is
-		 * destroyed. We call this again from kvm_destroy_realm() after
-		 * the RD is destroyed.
-		 */
-		return;
-	}
 	if (pgt) {
 		mmu->pgd_phys = 0;
 		mmu->pgt = NULL;

Sorry if I missed anything!

Thanks,
Wei-Lin Chang

> Thanks,
> Steve
> 
> > Thanks,
> > Wei-Lin Chang
> > 
> >>  	if (pgt) {
> >>  		mmu->pgd_phys = 0;
> >>  		mmu->pgt = NULL;
> >> diff --git a/arch/arm64/kvm/rmi.c b/arch/arm64/kvm/rmi.c
> >> index 700b8c935d29..1fd2c18f7381 100644
> >> --- a/arch/arm64/kvm/rmi.c
> >> +++ b/arch/arm64/kvm/rmi.c
> >> @@ -15,6 +15,19 @@
> >>  static unsigned long rmm_feat_reg0;
> >>  static unsigned long rmm_feat_reg1;
> >>  
> >> +#define RMM_RTT_BLOCK_LEVEL	2
> >> +#define RMM_RTT_MAX_LEVEL	3
> >> +
> >> +#define RMM_L2_BLOCK_SIZE	PMD_SIZE
> >> +
> >> +static inline unsigned long rmi_rtt_level_mapsize(int level)
> >> +{
> >> +	if (WARN_ON(level > RMM_RTT_MAX_LEVEL))
> >> +		return PAGE_SIZE;
> >> +
> >> +	return (1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(level));
> >> +}
> >> +
> >>  static bool rmi_has_feature(unsigned long feature)
> >>  {
> >>  	return !!u64_get_bits(rmm_feat_reg0, feature);
> >> @@ -189,6 +202,11 @@ u32 kvm_realm_ipa_limit(void)
> >>  	return u64_get_bits(rmm_feat_reg0, RMI_FEATURE_REGISTER_0_S2SZ);
> >>  }
> >>  
> >> +static int get_start_level(struct realm *realm)
> >> +{
> >> +	return 4 - stage2_pgtable_levels(realm->ia_bits);
> >> +}
> >> +
> >>  static int undelegate_range(phys_addr_t phys, unsigned long size)
> >>  {
> >>  	unsigned long ret;
> >> @@ -223,6 +241,131 @@ static int free_delegated_page(phys_addr_t phys)
> >>  	return 0;
> >>  }
> >>  
> >> +static void free_rtt(phys_addr_t phys)
> >> +{
> >> +	if (free_delegated_page(phys))
> >> +		return;
> >> +
> >> +	kvm_account_pgtable_pages(phys_to_virt(phys), -1);
> >> +}
> >> +
> >> +static int realm_rtt_destroy(struct realm *realm, unsigned long addr,
> >> +			     int level, phys_addr_t *rtt_granule,
> >> +			     unsigned long *next_addr)
> >> +{
> >> +	unsigned long out_rtt;
> >> +	int ret;
> >> +
> >> +	ret = rmi_rtt_destroy(virt_to_phys(realm->rd), addr, level,
> >> +			      &out_rtt, next_addr);
> >> +
> >> +	*rtt_granule = out_rtt;
> >> +
> >> +	return ret;
> >> +}
> >> +
> >> +static int realm_tear_down_rtt_level(struct realm *realm, int level,
> >> +				     unsigned long start, unsigned long end)
> >> +{
> >> +	ssize_t map_size;
> >> +	unsigned long addr, next_addr;
> >> +
> >> +	if (WARN_ON(level > RMM_RTT_MAX_LEVEL))
> >> +		return -EINVAL;
> >> +
> >> +	map_size = rmi_rtt_level_mapsize(level - 1);
> >> +
> >> +	for (addr = start; addr < end; addr = next_addr) {
> >> +		phys_addr_t rtt_granule;
> >> +		int ret;
> >> +		unsigned long align_addr = ALIGN(addr, map_size);
> >> +
> >> +		next_addr = ALIGN(addr + 1, map_size);
> >> +
> >> +		if (next_addr > end || align_addr != addr) {
> >> +			/*
> >> +			 * The target range is smaller than what this level
> >> +			 * covers, recurse deeper.
> >> +			 */
> >> +			ret = realm_tear_down_rtt_level(realm,
> >> +							level + 1,
> >> +							addr,
> >> +							min(next_addr, end));
> >> +			if (ret)
> >> +				return ret;
> >> +			continue;
> >> +		}
> >> +
> >> +		ret = realm_rtt_destroy(realm, addr, level,
> >> +					&rtt_granule, &next_addr);
> >> +
> >> +		switch (RMI_RETURN_STATUS(ret)) {
> >> +		case RMI_SUCCESS:
> >> +			free_rtt(rtt_granule);
> >> +			break;
> >> +		case RMI_ERROR_RTT:
> >> +			if (next_addr > addr) {
> >> +				/* Missing RTT, skip */
> >> +				break;
> >> +			}
> >> +			/*
> >> +			 * We tear down the RTT range for the full IPA
> >> +			 * space, after everything is unmapped. Also we
> >> +			 * descend down only if we cannot tear down a
> >> +			 * top level RTT. Thus RMM must be able to walk
> >> +			 * to the requested level. e.g., a block mapping
> >> +			 * exists at L1 or L2.
> >> +			 */
> >> +			if (WARN_ON(RMI_RETURN_INDEX(ret) != level))
> >> +				return -EBUSY;
> >> +			if (WARN_ON(level == RMM_RTT_MAX_LEVEL))
> >> +				return -EBUSY;
> >> +
> >> +			/*
> >> +			 * The table has active entries in it, recurse deeper
> >> +			 * and tear down the RTTs.
> >> +			 */
> >> +			next_addr = ALIGN(addr + 1, map_size);
> >> +			ret = realm_tear_down_rtt_level(realm,
> >> +							level + 1,
> >> +							addr,
> >> +							next_addr);
> >> +			if (ret)
> >> +				return ret;
> >> +			/*
> >> +			 * Now that the child RTTs are destroyed,
> >> +			 * retry at this level.
> >> +			 */
> >> +			next_addr = addr;
> >> +			break;
> >> +		default:
> >> +			WARN_ON(1);
> >> +			return -ENXIO;
> >> +		}
> >> +	}
> >> +
> >> +	return 0;
> >> +}
> >> +
> >> +static int realm_tear_down_rtt_range(struct realm *realm,
> >> +				     unsigned long start, unsigned long end)
> >> +{
> >> +	/*
> >> +	 * Root level RTTs can only be destroyed after the RD is destroyed. So
> >> +	 * tear down everything below the root level
> >> +	 */
> >> +	return realm_tear_down_rtt_level(realm, get_start_level(realm) + 1,
> >> +					 start, end);
> >> +}
> >> +
> >> +void kvm_realm_destroy_rtts(struct kvm *kvm)
> >> +{
> >> +	struct realm *realm = &kvm->arch.realm;
> >> +	unsigned int ia_bits = realm->ia_bits;
> >> +
> >> +	WARN_ON(realm_tear_down_rtt_range(realm, 0, (1UL << ia_bits)));
> >> +}
> >> +
> >>  void kvm_destroy_realm(struct kvm *kvm)
> >>  {
> >>  	struct realm *realm = &kvm->arch.realm;
> >> @@ -246,6 +389,8 @@ void kvm_destroy_realm(struct kvm *kvm)
> >>  	if (realm->rd) {
> >>  		phys_addr_t rd_phys = virt_to_phys(realm->rd);
> >>  
> >> +		kvm_realm_destroy_rtts(kvm);
> >> +
> >>  		if (WARN_ON(rmi_realm_destroy(rd_phys)))
> >>  			return;
> >>  		free_delegated_page(rd_phys);
> >> -- 
> >> 2.43.0
> >>
>