Re: [PATCH 07/11] KVM: arm64: CCA: Tear down RTTs
From: Steven Price
Date: Thu Nov 05 2026 - 11:34:40 EST
On 03/11/2026 15:20, Suzuki K Poulose wrote:
> On 02/11/2026 14:12, 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).
>>
>> Unmap the Realm IPA space once during stage-2 teardown. Defer destruction
>> of non-root RTTs until final Realm destruction, after REALM_TERMINATE and
>> before destroying the RD; root RTTs remain until the RD has gone away.
>>
>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>> ---
>> arch/arm64/include/asm/kvm_rmi.h | 9 ++
>> arch/arm64/kvm/mmu.c | 17 ++-
>> arch/arm64/kvm/rmi.c | 189 +++++++++++++++++++++++++++++++
>> 3 files changed, 214 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_rmi.h b/arch/arm64/include/
>> asm/kvm_rmi.h
>> index f1a1ed6b91f5..58d825e40bdf 100644
>> --- a/arch/arm64/include/asm/kvm_rmi.h
>> +++ b/arch/arm64/include/asm/kvm_rmi.h
>> @@ -57,6 +57,7 @@ enum realm_state {
>> * @sro: Preallocated SRO state context for Realm MMU operations
>> * @state: The lifetime state machine for the realm
>> * @ia_bits: Number of valid Input Address bits in the IPA
>> + * @stage2_unmapped: The Realm stage-2 mappings have been removed
>> */
>> struct realm {
>> void *rd;
>> @@ -70,6 +71,7 @@ struct realm {
>> struct rmi_sro_state *sro;
>> enum realm_state state;
>> unsigned int ia_bits;
>> + bool stage2_unmapped;
>> };
>> void kvm_init_rmi(void);
>> @@ -77,5 +79,12 @@ u32 kvm_rmm_ipa_limit(void);
>> int kvm_init_realm(struct kvm *kvm);
>> void kvm_destroy_realm(struct kvm *kvm);
>> +int kvm_realm_teardown_stage2(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 b049ea67d9af..51fda7b2ff7f 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1145,9 +1145,24 @@ 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);
>> +
>> + mutex_lock(&kvm->arch.config_lock);
>> + if (kvm_realm_state(kvm) == REALM_STATE_ACTIVE) {
>> + kvm_set_realm_state(kvm, REALM_STATE_DYING);
>> + WARN_ON(kvm_realm_teardown_stage2(kvm));
>> + }
>> + mutex_unlock(&kvm->arch.config_lock);
>> +}
>> +
>> void kvm_uninit_stage2_mmu(struct kvm *kvm)
>> {
>> - kvm_free_stage2_pgd(&kvm->arch.mmu);
>> + if (kvm_vm_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);
>> }
>> diff --git a/arch/arm64/kvm/rmi.c b/arch/arm64/kvm/rmi.c
>> index 45ecbaa467d1..f190143b33ba 100644
>> --- a/arch/arm64/kvm/rmi.c
>> +++ b/arch/arm64/kvm/rmi.c
>> @@ -11,6 +11,14 @@
>> #include <asm/kvm_pgtable.h>
>> #include <asm/virt.h>
>> +static inline unsigned long rmi_rtt_level_mapsize(int level)
>> +{
>> + if (WARN_ON(level > KVM_PGTABLE_LAST_LEVEL))
>> + return PAGE_SIZE;
>> +
>> + return (1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(level));
>> +}
>
>
> Now that we are using the same PAGE_SIZE as GRANULE size, we could
> replace the above helper with kvm_granule_size() ?
Makes sense - I added all these helpers when the RMM was always 4k page
size. Obviously I haven't yet managed to remove them all yet! ;)
>> static bool rmi_has_feature(int reg, unsigned long feature)
>> {
>> return !!u64_get_bits(rmi_feat_reg(reg), feature);
>> @@ -21,11 +29,180 @@ u32 kvm_rmm_ipa_limit(void)
>> return u64_get_bits(rmi_feat_reg(0), RMI_FEATURE_REGISTER_0_S2SZ);
>> }
>> +static int get_start_level(struct realm *realm)
>> +{
>> + return 4 - stage2_pgtable_levels(realm->ia_bits);
>> +}
>> +
>> +static void free_rtt(phys_addr_t phys)
>> +{
>> + if (rmi_free_delegated_page(phys))
>> + return;
>> +
>> + kvm_account_pgtable_pages(phys_to_virt(phys), -1);
>> +}
>> +
>
> ...
>
>> +
>> +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 > KVM_PGTABLE_LAST_LEVEL))
>> + return -EINVAL;
>> +
>> + map_size = rmi_rtt_level_mapsize(level - 1);
>> +
>> + for (addr = start; addr < end; addr = next_addr) {
>> + phys_addr_t rtt_granule;
>> + long ret;
>> + unsigned long align_addr = ALIGN(addr, map_size);
>
> We assume that the start is aligned to the kvm_granule_size(level - 1)
> and thus this works ? Should we enforce this before the loop ?
I'll add a WARN_ON() path to check this.
>> +
>> + next_addr = ALIGN(addr + 1, map_size);
>
> So that we can be sure we have (align_addr != next_addr).
>
>> +
>> + 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);
>> + if (ret < 0)
>> + return ret;
>> +
>> + switch (RMI_RESULT_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_RESULT_DATA_LEVEL(ret) != level))
>> + return -EBUSY;
>> + if (WARN_ON(level == KVM_PGTABLE_LAST_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);
>> +}
>> +
>> +static int realm_destroy_rtts(struct kvm *kvm)
>> +{
>> + struct realm *realm = &kvm->arch.realm;
>> + unsigned int ia_bits = realm->ia_bits;
>> +
>> + lockdep_assert_held(&kvm->arch.config_lock);
>
> Should we assert that realm->stage2_unmapped == true here, since we
> expect to not have any valid mappings ?
Will add.
>> +
>> + return realm_tear_down_rtt_range(realm, 0, (1UL << ia_bits));
>> +}
>> +
>> +static void realm_unmap_stage2(struct kvm *kvm)
>> +{
>> + struct realm *realm = &kvm->arch.realm;
>> +
>> + lockdep_assert_held(&kvm->arch.config_lock);
>> +
>> + if (realm->stage2_unmapped)
>> + return;
>> +
>> + write_lock(&kvm->mmu_lock);
>> + kvm_stage2_unmap_range(&kvm->arch.mmu, 0,
>> + BIT(realm->ia_bits - 1), true);
>
>
>> + write_unlock(&kvm->mmu_lock);
>
> Do we need this write_lock() ? IIUC, the Realm is dying and this is
> called from kvm_destroy_realm()/ kvm_free_stage2_pgd(). KVM assumes that
> stage2_destroy_range() is called without kvm->mmu_lock held (along with
> other relaxations). The comment above stage2_destroy_range() says:
I think you might be right that we don't *need* it. The main requirement
from the current code is the cond_resched_rwlock_write() call in
realm_unmap_xxxx_range(). So the options I see are:
1. Hold the lock even though we don't need to.
2. Force may_block to false even though it could be true.
3. Add an extra parameter to allow a call to cond_resched() rather than
cond_resched_rwlock_write().
My preference is to keep the code simple and hold the lock - do you see
any specific problem with taking the lock here?
Thanks,
Steve
>
> /*
> * Assume that @pgt is valid and unlinked from the KVM MMU to free the
> * page-table without taking the kvm_mmu_lock and without performing any
> * TLB invalidations.
> *
> * Also, the range of addresses can be large enough to cause need_resched
> * warnings, for instance on CONFIG_PREEMPT_NONE kernels. Hence, invoke
> * cond_resched() periodically to prevent hogging the CPU for a long time
> * and schedule something else, if required.
> */
>
> So, I believe we should be safe here without the lock ?
>
> Also, could we not call the realm unmap routine directly here, rather
> than going through the generic helper to get back to what we want ?
>
>
> Suzuki
>
>
>> +
>> + realm->stage2_unmapped = true;
>> +}
>> +
>> +int kvm_realm_teardown_stage2(struct kvm *kvm)
>> +{
>> + lockdep_assert_held(&kvm->arch.config_lock);
>> +
>> + realm_unmap_stage2(kvm);
>> + return 0;
>> +}
>> +
>> void kvm_destroy_realm(struct kvm *kvm)
>> {
>> struct realm *realm = &kvm->arch.realm;
>> size_t pgd_size = kvm_pgtable_stage2_pgd_size(kvm->arch.mmu.vtcr);
>> + guard(mutex)(&kvm->arch.config_lock);
>> +
>> if (!kvm_realm_is_created(kvm)) {
>> kfree(realm->sro);
>> realm->sro = NULL;
>> @@ -34,12 +211,24 @@ void kvm_destroy_realm(struct kvm *kvm)
>> kvm_set_realm_state(kvm, REALM_STATE_DYING);
>> + /*
>> + * REALM_DESTROY requires the realm to be non-live: all RECs must
>> have
>> + * been destroyed and the root RTTs must be empty. Unmap the IPA
>> space
>> + * and destroy any non-root RTTs before tearing down the RD. The
>> root
>> + * RTT pages are still owned by the RMM at this point, so keep
>> the KVM
>> + * pgtable alive until after REALM_DESTROY and undelegation.
>> + */
>> + realm_unmap_stage2(kvm);
>> +
>> if (realm->rd) {
>> phys_addr_t rd_phys = virt_to_phys(realm->rd);
>> if (WARN_ON(rmi_realm_terminate(rd_phys, realm->sro)))
>> return;
>> + if (WARN_ON(realm_destroy_rtts(kvm)))
>> + return;
>> +
>> if (WARN_ON(rmi_realm_destroy(rd_phys, realm->sro)))
>> return;
>> rmi_free_delegated_page(rd_phys);
>