Subject: Re: [PATCH v10 07/43] arm64: RME: ioctls to create and configure realms

From: Marc Zyngier <[email-masked]>
Date: Wed, 01 Oct 2025 16:36:52 +0100

On Wed, 20 Aug 2025 15:55:27 +0100,
Steven Price <[email-masked]> wrote:
> 
> Add the KVM_CAP_ARM_RME_CREATE_RD ioctl to create a realm. This involves
> delegating pages to the RMM to hold the Realm Descriptor (RD) and for
> the base level of the Realm Translation Tables (RTT). A VMID also need
> to be picked, since the RMM has a separate VMID address space a
> dedicated allocator is added for this purpose.
> 
> KVM_CAP_ARM_RME_CONFIG_REALM is provided to allow configuring the realm
> before it is created. Configuration options can be classified as:
> 
>  1. Parameters specific to the Realm stage2 (e.g. IPA Size, vmid, stage2
>     entry level, entry level RTTs, number of RTTs in start level, LPA2)
>     Most of these are not measured by RMM and comes from KVM book
>     keeping.
>
>  2. Parameters controlling "Arm Architecture features for the VM". (e.g.
>     SVE VL, PMU counters, number of HW BRPs/WPs), configured by the VMM
>     using the "user ID register write" mechanism. These will be
>     supported in the later patches.
> 
>  3. Parameters are not part of the core Arm architecture but defined
>     by the RMM spec (e.g. Hash algorithm for measurement,
>     Personalisation value). These are programmed via
>     KVM_CAP_ARM_RME_CONFIG_REALM.
> 
> For the IPA size there is the possibility that the RMM supports a
> different size to the IPA size supported by KVM for normal guests. At
> the moment the 'normal limit' is exposed by KVM_CAP_ARM_VM_IPA_SIZE and
> the IPA size is configured by the bottom bits of vm_type in
> KVM_CREATE_VM. This means that it isn't easy for the VMM to discover
> what IPA sizes are supported for Realm guests. Since the IPA is part of
> the measurement of the realm guest the current expectation is that the
> VMM will be required to pick the IPA size demanded by attestation and
> therefore simply failing if this isn't available is fine. An option
> would be to expose a new capability ioctl to obtain the RMM's maximum
> IPA size if this is needed in the future.

I think you must have something of the sort. Configuring the IPA size
is one of the basic things a VMM performs, querying the host for that
information. Implicit IPA size prevents the basics of what we have
userspace for: management of the IPA space. Otherwise, how can the
guest know where to place its I/O, for example?

> 
> Co-developed-by: Suzuki K Poulose <[email-masked]>
> Signed-off-by: Suzuki K Poulose <[email-masked]>
> Signed-off-by: Steven Price <[email-masked]>
> Reviewed-by: Gavin Shan <[email-masked]>
> ---
> Changes since v9:
>  * Avoid walking the stage 2 page tables when destroying the realm -
>    the real ones are not accessible to the non-secure world, and the RMM
>    may leave junk in the physical pages when returning them.
>  * Fix an error path in realm_create_rd() to actually return an error value.
> Changes since v8:
>  * Fix free_delegated_granule() to not call kvm_account_pgtable_pages();
>    a separate wrapper will be introduced in a later patch to deal with
>    RTTs.
>  * Minor code cleanups following review.
> Changes since v7:
>  * Minor code cleanup following Gavin's review.
> Changes since v6:
>  * Separate RMM RTT calculations from host PAGE_SIZE. This allows the
>    host page size to be larger than 4k while still communicating with an
>    RMM which uses 4k granules.
> Changes since v5:
>  * Introduce free_delegated_granule() to replace many
>    undelegate/free_page() instances and centralise the comment on
>    leaking when the undelegate fails.
>  * Several other minor improvements suggested by reviews - thanks for
>    the feedback!
> Changes since v2:
>  * Improved commit description.
>  * Improved return failures for rmi_check_version().
>  * Clear contents of PGD after it has been undelegated in case the RMM
>    left stale data.
>  * Minor changes to reflect changes in previous patches.
> ---
>  arch/arm64/include/asm/kvm_emulate.h |   5 +
>  arch/arm64/include/asm/kvm_rme.h     |  19 ++
>  arch/arm64/kvm/arm.c                 |  16 ++
>  arch/arm64/kvm/mmu.c                 |  24 +-
>  arch/arm64/kvm/rme.c                 | 322 +++++++++++++++++++++++++++
>  5 files changed, 383 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index ab4093e41c4b..f429ad704850 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -687,6 +687,11 @@ static inline enum realm_state kvm_realm_state(struct kvm *kvm)
>  	return READ_ONCE(kvm->arch.realm.state);
>  }
>  
> +static inline bool kvm_realm_is_created(struct kvm *kvm)
> +{
> +	return kvm_is_realm(kvm) && kvm_realm_state(kvm) != REALM_STATE_NONE;
> +}
> +
>  static inline bool vcpu_is_rec(struct kvm_vcpu *vcpu)
>  {
>  	return false;
> diff --git a/arch/arm64/include/asm/kvm_rme.h b/arch/arm64/include/asm/kvm_rme.h
> index 9c8a0b23e0e4..5dc1915de891 100644
> --- a/arch/arm64/include/asm/kvm_rme.h
> +++ b/arch/arm64/include/asm/kvm_rme.h
> @@ -6,6 +6,8 @@
>  #ifndef __ASM_KVM_RME_H
>  #define __ASM_KVM_RME_H
>  
> +#include <uapi/linux/kvm.h>
> +

Why do you need to include a uapi-specific file for something that is
deeply arch-specific? This is a recipe for circular dependencies that
are horrible to solve.

>  /**
>   * enum realm_state - State of a Realm
>   */
> @@ -46,11 +48,28 @@ enum realm_state {
>   * struct realm - Additional per VM data for a Realm
>   *
>   * @state: The lifetime state machine for the realm
> + * @rd: Kernel mapping of the Realm Descriptor (RD)
> + * @params: Parameters for the RMI_REALM_CREATE command
> + * @num_aux: The number of auxiliary pages required by the RMM
> + * @vmid: VMID to be used by the RMM for the realm
> + * @ia_bits: Number of valid Input Address bits in the IPA
>   */
>  struct realm {
>  	enum realm_state state;
> +
> +	void *rd;
> +	struct realm_params *params;
> +
> +	unsigned long num_aux;
> +	unsigned int vmid;
> +	unsigned int ia_bits;
>  };
>  
>  void kvm_init_rme(void);
> +u32 kvm_realm_ipa_limit(void);
> +
> +int kvm_realm_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap);
> +int kvm_init_realm_vm(struct kvm *kvm);
> +void kvm_destroy_realm(struct kvm *kvm);
>  
>  #endif /* __ASM_KVM_RME_H */
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 76177c56f1ef..1acee3861e55 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -136,6 +136,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		}
>  		mutex_unlock(&kvm->lock);
>  		break;
> +	case KVM_CAP_ARM_RME:
> +		mutex_lock(&kvm->lock);
> +		r = kvm_realm_enable_cap(kvm, cap);
> +		mutex_unlock(&kvm->lock);
> +		break;
>  	default:
>  		break;
>  	}
> @@ -198,6 +203,13 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  
>  	bitmap_zero(kvm->arch.vcpu_features, KVM_VCPU_MAX_FEATURES);
>  
> +	/* Initialise the realm bits after the generic bits are enabled */
> +	if (kvm_is_realm(kvm)) {
> +		ret = kvm_init_realm_vm(kvm);
> +		if (ret)
> +			goto err_free_cpumask;
> +	}
> +
>  	return 0;
>  
>  err_free_cpumask:
> @@ -257,6 +269,7 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
>  	kvm_unshare_hyp(kvm, kvm + 1);
>  
>  	kvm_arm_teardown_hypercalls(kvm);
> +	kvm_destroy_realm(kvm);
>  }
>  
>  static bool kvm_has_full_ptr_auth(void)
> @@ -417,6 +430,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  		else
>  			r = kvm_supports_cacheable_pfnmap();
>  		break;
> +	case KVM_CAP_ARM_RME:
> +		r = static_key_enabled(&kvm_rme_is_available);
> +		break;
>  
>  	default:
>  		r = 0;
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 1c78864767c5..de10dbde4761 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -871,12 +871,16 @@ static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = {
>  	.icache_inval_pou	= invalidate_icache_guest_page,
>  };
>  
> -static int kvm_init_ipa_range(struct kvm_s2_mmu *mmu, unsigned long type)
> +static int kvm_init_ipa_range(struct kvm *kvm,
> +			      struct kvm_s2_mmu *mmu, unsigned long type)
>  {
>  	u32 kvm_ipa_limit = get_kvm_ipa_limit();
>  	u64 mmfr0, mmfr1;
>  	u32 phys_shift;
>  
> +	if (kvm_is_realm(kvm))
> +		kvm_ipa_limit = kvm_realm_ipa_limit();
> +
>  	if (type & ~KVM_VM_TYPE_ARM_IPA_SIZE_MASK)
>  		return -EINVAL;
>  
> @@ -941,7 +945,7 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
>  		return -EINVAL;
>  	}
>  
> -	err = kvm_init_ipa_range(mmu, type);
> +	err = kvm_init_ipa_range(kvm, mmu, type);
>  	if (err)
>  		return err;
>  
> @@ -1067,6 +1071,19 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>  	struct kvm_pgtable *pgt = NULL;
>  
>  	write_lock(&kvm->mmu_lock);
> +	if (kvm_is_realm(kvm) &&
> +	    (kvm_realm_state(kvm) != REALM_STATE_DEAD &&
> +	     kvm_realm_state(kvm) != REALM_STATE_NONE)) {
> +		/* Tearing down RTTs will be added in a later patch */
> +		write_unlock(&kvm->mmu_lock);
> +
> +		/*
> +		 * 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;
> +	}

Please move S2 management outside of this patch. It's hard to follow
what is happening when this sort of stuff is split over multiple patches.

>  	pgt = mmu->pgt;
>  	if (pgt) {
>  		mmu->pgd_phys = 0;
> @@ -1076,7 +1093,8 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>  	write_unlock(&kvm->mmu_lock);
>  
>  	if (pgt) {
> -		KVM_PGT_FN(kvm_pgtable_stage2_destroy)(pgt);
> +		if (!kvm_is_realm(kvm))
> +			KVM_PGT_FN(kvm_pgtable_stage2_destroy)(pgt);

having to do stuff like this is horrid. Please provide a backend to
the KVM_PGT_FN() helper instead.

>  		kfree(pgt);
>  	}
>  }
> diff --git a/arch/arm64/kvm/rme.c b/arch/arm64/kvm/rme.c
> index 67cf2d94cb2d..4a2c557bf0a7 100644
> --- a/arch/arm64/kvm/rme.c
> +++ b/arch/arm64/kvm/rme.c
> @@ -5,9 +5,23 @@
>  
>  #include <linux/kvm_host.h>
>  
> +#include <asm/kvm_emulate.h>
> +#include <asm/kvm_mmu.h>
>  #include <asm/rmi_cmds.h>
>  #include <asm/virt.h>
>  
> +#include <asm/kvm_pgtable.h>
> +
> +static unsigned long rmm_feat_reg0;
> +
> +#define RMM_PAGE_SHIFT		12
> +#define RMM_PAGE_SIZE		BIT(RMM_PAGE_SHIFT)
> +
> +static bool rme_has_feature(unsigned long feature)
> +{
> +	return !!u64_get_bits(rmm_feat_reg0, feature);
> +}
> +
>  static int rmi_check_version(void)
>  {
>  	struct arm_smccc_res res;
> @@ -42,6 +56,308 @@ static int rmi_check_version(void)
>  	return 0;
>  }
>  
> +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)
> +{
> +	/*
> +	 * Open coded version of 4 - stage2_pgtable_levels(ia_bits) but using
> +	 * the RMM's page size rather than the host's.
> +	 */
> +	return 4 - ((realm->ia_bits - 8) / (RMM_PAGE_SHIFT - 3));

Why 8?

> +}
> +
> +static int free_delegated_granule(phys_addr_t phys)
> +{
> +	if (WARN_ON(rmi_granule_undelegate(phys))) {
> +		/* Undelegate failed: leak the page */
> +		return -EBUSY;
> +	}
> +
> +	free_page((unsigned long)phys_to_virt(phys));
> +
> +	return 0;
> +}
> +
> +/* Calculate the number of s2 root rtts needed */
> +static int realm_num_root_rtts(struct realm *realm)
> +{
> +	unsigned int ipa_bits = realm->ia_bits;
> +	unsigned int levels = 4 - get_start_level(realm);
> +	unsigned int sl_ipa_bits = levels * (RMM_PAGE_SHIFT - 3) +
> +				   RMM_PAGE_SHIFT;
> +
> +	if (sl_ipa_bits >= ipa_bits)
> +		return 1;
> +
> +	return 1 << (ipa_bits - sl_ipa_bits);

Don't we already have similar things in pgtables.c? It has a strong
feel of deja-vu.

> +}
> +
> +static int realm_create_rd(struct kvm *kvm)
> +{
> +	struct realm *realm = &kvm->arch.realm;
> +	struct realm_params *params = realm->params;
> +	void *rd = NULL;
> +	phys_addr_t rd_phys, params_phys;
> +	size_t pgd_size = kvm_pgtable_stage2_pgd_size(kvm->arch.mmu.vtcr);
> +	int i, r;
> +	int rtt_num_start;
> +
> +	realm->ia_bits = VTCR_EL2_IPA(kvm->arch.mmu.vtcr);
> +	rtt_num_start = realm_num_root_rtts(realm);
> +
> +	if (WARN_ON(realm->rd || !realm->params))
> +		return -EEXIST;
> +
> +	if (pgd_size / RMM_PAGE_SIZE < rtt_num_start)
> +		return -EINVAL;
> +
> +	rd = (void *)__get_free_page(GFP_KERNEL);
> +	if (!rd)
> +		return -ENOMEM;
> +
> +	rd_phys = virt_to_phys(rd);
> +	if (rmi_granule_delegate(rd_phys)) {
> +		r = -ENXIO;
> +		goto free_rd;
> +	}
> +
> +	for (i = 0; i < pgd_size; i += RMM_PAGE_SIZE) {
> +		phys_addr_t pgd_phys = kvm->arch.mmu.pgd_phys + i;
> +
> +		if (rmi_granule_delegate(pgd_phys)) {
> +			r = -ENXIO;
> +			goto out_undelegate_tables;
> +		}
> +	}
> +
> +	params->s2sz = VTCR_EL2_IPA(kvm->arch.mmu.vtcr);
> +	params->rtt_level_start = get_start_level(realm);
> +	params->rtt_num_start = rtt_num_start;
> +	params->rtt_base = kvm->arch.mmu.pgd_phys;
> +	params->vmid = realm->vmid;
> +
> +	params_phys = virt_to_phys(params);
> +
> +	if (rmi_realm_create(rd_phys, params_phys)) {
> +		r = -ENXIO;
> +		goto out_undelegate_tables;
> +	}
> +
> +	if (WARN_ON(rmi_rec_aux_count(rd_phys, &realm->num_aux))) {
> +		WARN_ON(rmi_realm_destroy(rd_phys));
> +		r = -ENXIO;
> +		goto out_undelegate_tables;
> +	}
> +
> +	realm->rd = rd;
> +
> +	return 0;
> +
> +out_undelegate_tables:
> +	while (i > 0) {
> +		i -= RMM_PAGE_SIZE;
> +
> +		phys_addr_t pgd_phys = kvm->arch.mmu.pgd_phys + i;
> +
> +		if (WARN_ON(rmi_granule_undelegate(pgd_phys))) {
> +			/* Leak the pages if they cannot be returned */
> +			kvm->arch.mmu.pgt = NULL;
> +			break;
> +		}
> +	}
> +	if (WARN_ON(rmi_granule_undelegate(rd_phys))) {
> +		/* Leak the page if it isn't returned */
> +		return r;
> +	}
> +free_rd:
> +	free_page((unsigned long)rd);
> +	return r;
> +}
> +
> +/* Protects access to rme_vmid_bitmap */
> +static DEFINE_SPINLOCK(rme_vmid_lock);
> +static unsigned long *rme_vmid_bitmap;
> +
> +static int rme_vmid_init(void)
> +{
> +	unsigned int vmid_count = 1 << kvm_get_vmid_bits();
> +
> +	rme_vmid_bitmap = bitmap_zalloc(vmid_count, GFP_KERNEL);
> +	if (!rme_vmid_bitmap) {
> +		kvm_err("%s: Couldn't allocate rme vmid bitmap\n", __func__);
> +		return -ENOMEM;
> +	}
> +
> +	return 0;
> +}

Why do we need a VMID allocator? This is a different translation
regime, which won't intersect with the host's. Surely the RMM can
maintain its own?

> +
> +static int rme_vmid_reserve(void)
> +{
> +	int ret;
> +	unsigned int vmid_count = 1 << kvm_get_vmid_bits();
> +
> +	spin_lock(&rme_vmid_lock);
> +	ret = bitmap_find_free_region(rme_vmid_bitmap, vmid_count, 0);
> +	spin_unlock(&rme_vmid_lock);
> +
> +	return ret;
> +}
> +
> +static void rme_vmid_release(unsigned int vmid)
> +{
> +	spin_lock(&rme_vmid_lock);
> +	bitmap_release_region(rme_vmid_bitmap, vmid, 0);
> +	spin_unlock(&rme_vmid_lock);
> +}
> +
> +static int kvm_create_realm(struct kvm *kvm)
> +{
> +	struct realm *realm = &kvm->arch.realm;
> +	int ret;
> +
> +	if (kvm_realm_is_created(kvm))
> +		return -EEXIST;
> +
> +	ret = rme_vmid_reserve();
> +	if (ret < 0)
> +		return ret;
> +	realm->vmid = ret;
> +
> +	ret = realm_create_rd(kvm);
> +	if (ret) {
> +		rme_vmid_release(realm->vmid);
> +		return ret;
> +	}
> +
> +	WRITE_ONCE(realm->state, REALM_STATE_NEW);
> +
> +	/* The realm is up, free the parameters.  */
> +	free_page((unsigned long)realm->params);
> +	realm->params = NULL;
> +
> +	return 0;
> +}
> +
> +static int config_realm_hash_algo(struct realm *realm,
> +				  struct arm_rme_config *cfg)
> +{
> +	switch (cfg->hash_algo) {
> +	case ARM_RME_CONFIG_HASH_ALGO_SHA256:
> +		if (!rme_has_feature(RMI_FEATURE_REGISTER_0_HASH_SHA_256))
> +			return -EINVAL;
> +		break;
> +	case ARM_RME_CONFIG_HASH_ALGO_SHA512:
> +		if (!rme_has_feature(RMI_FEATURE_REGISTER_0_HASH_SHA_512))
> +			return -EINVAL;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +	realm->params->hash_algo = cfg->hash_algo;
> +	return 0;
> +}

What is the purpose of this?

Overall, there is a lot here that needs to be explained, because I
don't immediately understand what this stuff is all about. This file
needs a large comment at the front explaining what is all that for.

Paste the whole spec in if that helps.

	M.

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