Re: [PATCH v7 02/24] KVM: arm64: Donate MMIO to the hypervisor

[Sebastian Ene <sebastianene@google.com>]

On Wed, Jul 15, 2026 at 11:58:43AM +0000, Mostafa Saleh wrote:
> Add a function to donate MMIO to the hypervisor so IOMMU hypervisor
> drivers can protect and access the MMIO of IOMMUs.
> 
> As donating MMIO is very rare, and we don’t need to encode the full
> state, it’s reasonable to have a separate function to do this.
> It will init the host s2 page table with an invalid leaf with the owner ID
> to prevent the host from mapping the page on faults.
> 
> Also, prevent kvm_pgtable_stage2_unmap() from removing owner ID from
> stage-2 PTEs, as this can be triggered from recycle logic under memory
> pressure. There is no code relying on this, as all ownership changes is
> done via kvm_pgtable_stage2_set_owner()
> 
> For the error path in IOMMU drivers, add a function to donate MMIO
> back from hyp to host. However, that leaks the hypervisor virtual
> address range which should be acceptable as this is quite rare and
> it matches the behaviour of fix_map/block.
> 
> Signed-off-by: Mostafa Saleh <smostafa@google.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/mem_protect.h |  7 ++
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c         | 91 ++++++++++++++++++-
>  arch/arm64/kvm/hyp/pgtable.c                  | 11 +--
>  3 files changed, 102 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> index 29935c7da1de..51b0eb3844a9 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> @@ -36,6 +36,13 @@ int __pkvm_guest_share_host(struct pkvm_hyp_vcpu *vcpu, u64 gfn);
>  int __pkvm_guest_unshare_host(struct pkvm_hyp_vcpu *vcpu, u64 gfn);
>  int __pkvm_host_unshare_hyp(u64 pfn);
>  int __pkvm_host_donate_hyp(u64 pfn, u64 nr_pages);
> +/*
> + * Donate MMIO range to the hypervisor, it will be mapped in the hypervisor's
> + * private range and unmapped from the host stage-2.
> + */
> +int __pkvm_host_donate_hyp_mmio(phys_addr_t addr, size_t size, unsigned long *haddr);
> +/* Remaps MMIO range in the host, typically used in error path. */
> +int __pkvm_hyp_donate_host_mmio(phys_addr_t addr, size_t size);
>  int __pkvm_hyp_donate_host(u64 pfn, u64 nr_pages);
>  int __pkvm_host_share_ffa(u64 pfn, u64 nr_pages);
>  int __pkvm_host_unshare_ffa(u64 pfn, u64 nr_pages);
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index 4e329e39a695..d803b3dd4cb4 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -378,7 +378,11 @@ static int host_stage2_unmap_dev_all(void)
>  	u64 addr = 0;
>  	int i, ret;
>  
> -	/* Unmap all non-memory regions to recycle the pages */
> +	/*
> +	 * Unmap all non-memory regions to recycle the pages.
> +	 * That relies on kvm_pgtable_stage2_unmap() not clearing
> +	 * counted PTEs which include hypervisor MMIO.
> +	 */
>  	for (i = 0; i < hyp_memblock_nr; i++, addr = reg->base + reg->size) {
>  		reg = &hyp_memory[i];
>  		ret = kvm_pgtable_stage2_unmap(pgt, addr, reg->base - addr);
> @@ -1119,6 +1123,91 @@ int __pkvm_host_donate_hyp(u64 pfn, u64 nr_pages)
>  	return ret;
>  }
>  
> +int __pkvm_host_donate_hyp_mmio(phys_addr_t addr, size_t size, unsigned long *haddr)
> +{
> +	kvm_pte_t pte;
> +	u64 offset;
> +	int ret;
> +
> +	/* Only before de-privilege. */
> +	if (static_branch_unlikely(&kvm_protected_mode_initialized))
> +		return -EPERM;
> +
> +	if (!PAGE_ALIGNED(addr | size) ||
> +	    !pfn_range_is_valid(hyp_phys_to_pfn(addr), size >> PAGE_SHIFT))
> +		return -EINVAL;
> +
> +	ret = __pkvm_create_private_mapping(addr, size, PAGE_HYP_DEVICE, haddr);

> +	if (ret)
> +		return ret;
> +
> +	host_lock_component();
> +	for (offset = 0; offset < size; offset += PAGE_SIZE) {
> +		if (addr_is_memory(addr + offset)) {
> +			ret = -EINVAL;
> +			goto unlock;

If this fails we are left with the mapping inside the hyp because the unlock
doesn't destroy the private mapping. Is this intended ?

> +		}
> +		ret = kvm_pgtable_get_leaf(&host_mmu.pgt, addr + offset, &pte, NULL);
> +		if (ret)
> +			goto unlock;
> +		if (pte && !kvm_pte_valid(pte)) {
> +			ret = -EPERM;
> +			goto unlock;
> +		}
> +	}
> +	/*
> +	 * We set HYP as the owner of the MMIO pages in the host stage-2, for:
> +	 * - host aborts: host_stage2_adjust_range() would fail for invalid non zero PTEs.
> +	 * - recycle under memory pressure: host_stage2_unmap_dev_all() would call
> +	 *   kvm_pgtable_stage2_unmap() which will not clear non zero invalid ptes (counted).
> +	 * - other MMIO donation: Would fail as we check that the PTE is valid or empty.
> +	 */
> +	ret = host_stage2_try(kvm_pgtable_stage2_annotate, &host_mmu.pgt,
> +			      addr, size, &host_s2_pool,
> +			      KVM_HOST_INVALID_PTE_TYPE_DONATION,
> +			      FIELD_PREP(KVM_HOST_DONATION_PTE_OWNER_MASK, PKVM_ID_HYP));
> +unlock:
> +	host_unlock_component();
> +	return ret;
> +}
> +
> +int __pkvm_hyp_donate_host_mmio(phys_addr_t addr, size_t size)

This function seems to only update the host stage-2 annotation but it
doesn't destroy the hyp mapping.

I was looking to make use of this patch in an upcoming posting for the
v2 ITS hardening but in my case I don't need the private VA range
creation.

Thanks,
Sebastian

> +{
> +	kvm_pte_t pte;
> +	u64 offset;
> +	int ret = 0;
> +
> +	if (static_branch_unlikely(&kvm_protected_mode_initialized))
> +		return -EPERM;
> +
> +	if (!PAGE_ALIGNED(addr | size) ||
> +	    !pfn_range_is_valid(hyp_phys_to_pfn(addr), size >> PAGE_SHIFT))
> +		return -EINVAL;
> +
> +	host_lock_component();
> +	for (offset = 0; offset < size; offset += PAGE_SIZE) {
> +		if (addr_is_memory(addr + offset)) {
> +			ret = -EINVAL;
> +			goto unlock;
> +		}
> +		ret = kvm_pgtable_get_leaf(&host_mmu.pgt, addr + offset, &pte, NULL);
> +		if (ret)
> +			goto unlock;
> +		if (!pte || kvm_pte_valid(pte)) {
> +			ret = -EINVAL;
> +			goto unlock;
> +		}
> +		if (FIELD_GET(KVM_HOST_DONATION_PTE_OWNER_MASK, pte) != PKVM_ID_HYP) {
> +			ret = -EPERM;
> +			goto unlock;
> +		}
> +	}
> +	WARN_ON(host_stage2_idmap_locked(addr, size, PKVM_HOST_MMIO_PROT));
> +unlock:
> +	host_unlock_component();
> +	return ret;
> +}
> +
>  int __pkvm_hyp_donate_host(u64 pfn, u64 nr_pages)
>  {
>  	u64 phys = hyp_pfn_to_phys(pfn);
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 91a7dfad6686..3073184cf6ad 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1161,13 +1161,12 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
>  	kvm_pte_t *childp = NULL;
>  	bool need_flush = false;
>  
> -	if (!kvm_pte_valid(ctx->old)) {
> -		if (stage2_pte_is_counted(ctx->old)) {
> -			kvm_clear_pte(ctx->ptep);
> -			mm_ops->put_page(ctx->ptep);
> -		}
> +	/*
> +	 * That also ignores stage2_pte_is_counted() instead of clearing
> +	 * the PTE as the MMIO can be owned by the hypervisor.
> +	 */
> +	if (!kvm_pte_valid(ctx->old))
>  		return 0;
> -	}
>  
>  	if (kvm_pte_table(ctx->old, ctx->level)) {
>  		childp = kvm_pte_follow(ctx->old, mm_ops);
> -- 
> 2.55.0.141.g00534a21ce-goog
>