Re: [PATCH v14 29/44] arm64: RMI: Runtime faulting of memory

[Gavin Shan <gshan@redhat.com>]

Hi Steve,

On 5/13/26 11:17 PM, Steven Price wrote:
> At runtime if the realm guest accesses memory which hasn't yet been
> mapped then KVM needs to either populate the region or fault the guest.
> 
> For memory in the lower (protected) region of IPA a fresh page is
> provided to the RMM which will zero the contents. For memory in the
> upper (shared) region of IPA, the memory from the memslot is mapped
> into the realm VM non secure.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> Changes since v13:
>   * Numerous changes due to rebasing.
>   * Fix addr_range_desc() to encode the correct block size.
> Changes since v12:
>   * Switch to RMM v2.0 range based APIs.
> Changes since v11:
>   * Adapt to upstream changes.
> Changes since v10:
>   * RME->RMI renaming.
>   * Adapt to upstream gmem changes.
> Changes since v9:
>   * Fix call to kvm_stage2_unmap_range() in kvm_free_stage2_pgd() to set
>     may_block to avoid stall warnings.
>   * Minor coding style fixes.
> Changes since v8:
>   * Propagate the may_block flag.
>   * Minor comments and coding style changes.
> Changes since v7:
>   * Remove redundant WARN_ONs for realm_create_rtt_levels() - it will
>     internally WARN when necessary.
> Changes since v6:
>   * Handle PAGE_SIZE being larger than RMM granule size.
>   * Some minor renaming following review comments.
> Changes since v5:
>   * Reduce use of struct page in preparation for supporting the RMM
>     having a different page size to the host.
>   * Handle a race when delegating a page where another CPU has faulted on
>     a the same page (and already delegated the physical page) but not yet
>     mapped it. In this case simply return to the guest to either use the
>     mapping from the other CPU (or refault if the race is lost).
>   * The changes to populate_par_region() are moved into the previous
>     patch where they belong.
> Changes since v4:
>   * Code cleanup following review feedback.
>   * Drop the PTE_SHARED bit when creating unprotected page table entries.
>     This is now set by the RMM and the host has no control of it and the
>     spec requires the bit to be set to zero.
> Changes since v2:
>   * Avoid leaking memory if failing to map it in the realm.
>   * Correctly mask RTT based on LPA2 flag (see rtt_get_phys()).
>   * Adapt to changes in previous patches.
> ---
>   arch/arm64/include/asm/kvm_emulate.h |   8 ++
>   arch/arm64/include/asm/kvm_rmi.h     |  12 ++
>   arch/arm64/kvm/mmu.c                 | 128 ++++++++++++++++----
>   arch/arm64/kvm/rmi.c                 | 173 +++++++++++++++++++++++++++
>   4 files changed, 301 insertions(+), 20 deletions(-)
> 

[...]

> @@ -1604,27 +1641,52 @@ static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
>   	bool write_fault, exec_fault;
>   	enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_SHARED;
>   	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> -	struct kvm_pgtable *pgt = s2fd->vcpu->arch.hw_mmu->pgt;
> +	struct kvm_vcpu *vcpu = s2fd->vcpu;
> +	struct kvm_pgtable *pgt = vcpu->arch.hw_mmu->pgt;
> +	gpa_t gpa = kvm_gpa_from_fault(vcpu->kvm, s2fd->fault_ipa);
>   	unsigned long mmu_seq;
>   	struct page *page;
> -	struct kvm *kvm = s2fd->vcpu->kvm;
> +	struct kvm *kvm = vcpu->kvm;
>   	void *memcache;
>   	kvm_pfn_t pfn;
>   	gfn_t gfn;
>   	int ret;
>   
> -	memcache = get_mmu_memcache(s2fd->vcpu);
> -	ret = topup_mmu_memcache(s2fd->vcpu, memcache);
> +	if (kvm_is_realm(vcpu->kvm)) {
> +		/* check for memory attribute mismatch */
> +		bool is_priv_gfn = kvm_mem_is_private(kvm, gpa >> PAGE_SHIFT);
> +		/*
> +		 * For Realms, the shared address is an alias of the private
> +		 * PA with the top bit set. Thus if the fault address matches
> +		 * the GPA then it is the private alias.
> +		 */
> +		bool is_priv_fault = (gpa == s2fd->fault_ipa);
> +
> +		if (is_priv_gfn != is_priv_fault) {
> +			kvm_prepare_memory_fault_exit(vcpu, gpa, PAGE_SIZE,
> +						      kvm_is_write_fault(vcpu),
> +						      false,
> +						      is_priv_fault);
> +			/*
> +			 * KVM_EXIT_MEMORY_FAULT requires an return code of
> +			 * -EFAULT, see the API documentation
> +			 */
> +			return -EFAULT;
> +		}
> +	}
> +

For a Realm, gmem_abort() is called by kvm_handle_guest_abort() only when
we're faulting in the private (protected) space.

     if (kvm_slot_has_gmem(memslot) && !shared_ipa_fault(vcpu->kvm, fault_ipa))
         ret = gmem_abort(&s2fd);
     else
         ret = user_mem_abort(&s2fd);

With the condition, this block of code can be simplied to handle conversion
(shared -> private) instead of both directions.

     /* Convert the shared address to the private adress for Realm */
     if (kvm_is_realm(vcpu->kvm) &&
         !kvm_mem_is_private(kvm, gpa >> PAGE_SHIFT)) {
         /*
          * KVM_EXIT_MEMORY_FAULT requires an return code of
          * -EFAULT, see the API documentation
          */
         kvm_prepare_memory_fault_exit(vcpu, gpa, PAGE_SIZE,
                                       kvm_is_write_fault(vcpu),
                                       false, true);
         return -EFAULT;
     }


[...]

> @@ -2396,7 +2475,7 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
>   				!write_fault &&
>   				!kvm_vcpu_trap_is_exec_fault(vcpu));
>   
> -		if (kvm_slot_has_gmem(memslot))
> +		if (kvm_slot_has_gmem(memslot) && !shared_ipa_fault(vcpu->kvm, fault_ipa))
>   			ret = gmem_abort(&s2fd);
>   		else
>   			ret = user_mem_abort(&s2fd);
gmem_abort() is only called for faults in the protected (private) space.

Thanks,
Gavin