Re: [PATCH 01/14] KVM: arm64: Donate MMIO to the hypervisor

Fuad Tabba
2026-03-12T17:57:55Z

Hi Sebastian (and Mostafa),

On Tue, 10 Mar 2026 at 12:49, Sebastian Ene <sebastianene@google.com> wrote:
>
> From: Mostafa Saleh <smostafa@google.com>
>
> Add a function to donate MMIO to the hypervisor so IOMMU hypervisor
> drivers can use that to protect the MMIO of IOMMU.
> The initial attempt to implement this was to have a new flag to
> "___pkvm_host_donate_hyp" to accept MMIO. However that had many problems,
> it was quite intrusive for host/hyp to check/set page state to make it
> aware of MMIO and to encode the state in the page table in that case.
> Which is called in paths that can be sensitive to performance (FFA, VMs..)
>
> 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 error path in IOMMU drivers, add a function to donate MMIO back
> from hyp to host.
>
> Signed-off-by: Mostafa Saleh <smostafa@google.com>
> ---
> arch/arm64/kvm/hyp/include/nvhe/mem_protect.h | 2 +
> arch/arm64/kvm/hyp/nvhe/mem_protect.c | 90 +++++++++++++++++++
> arch/arm64/kvm/hyp/pgtable.c | 9 +-
> 3 files changed, 94 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 5f9d56754e39..8b617e6fc0e0 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> @@ -31,6 +31,8 @@ enum pkvm_component_id {
> };
>
> extern unsigned long hyp_nr_cpus;
> +int __pkvm_host_donate_hyp_mmio(u64 pfn);
> +int __pkvm_hyp_donate_host_mmio(u64 pfn);
>
> int __pkvm_prot_finalize(void);
> int __pkvm_host_share_hyp(u64 pfn);
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index 38f66a56a766..0808367c52e5 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -784,6 +784,96 @@ int __pkvm_host_unshare_hyp(u64 pfn)
> return ret;
> }
>
> +int __pkvm_host_donate_hyp_mmio(u64 pfn)
> +{
> + u64 phys = hyp_pfn_to_phys(pfn);
> + void *virt = __hyp_va(phys);
> + int ret;
> + kvm_pte_t pte;

nit: prefer reverse-christmas tree ordering for declarations.

> +
> + if (addr_is_memory(phys))
> + return -EINVAL;
> +
> + host_lock_component();
> + hyp_lock_component();
> +
> + ret = kvm_pgtable_get_leaf(&host_mmu.pgt, phys, &pte, NULL);
> + if (ret)
> + goto unlock;
> +
> + if (pte && !kvm_pte_valid(pte)) {
> + ret = -EPERM;
> + goto unlock;
> + }
> +
> + ret = kvm_pgtable_get_leaf(&pkvm_pgtable, (u64)virt, &pte, NULL);
> + if (ret)
> + goto unlock;
> + if (pte) {
> + ret = -EBUSY;
> + goto unlock;
> + }
> +
> + ret = pkvm_create_mappings_locked(virt, virt + PAGE_SIZE, PAGE_HYP_DEVICE);
> + if (ret)
> + 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.
> + */

Rephrasing this to see if I got this right. Because MMIO pages are not
tracked in the hyp_vmemmap, the host's stage-2 PTE serves as the sole
record of ownership. We encode PKVM_ID_HYP into an invalid PTE (a
"counted" PTE) to ensure:
- Fault Handling: host_stage2_adjust_range() will explicitly fail if
the host attempts to legitimately map or adjust the protected range.
- Reclaim Protection: Under memory pressure,
host_stage2_unmap_dev_all() will not erase this ownership marker,
because you changed stage2_unmap_walker() to preserve counted PTEs.
- Exclusive Ownership: Subsequent donation attempts (to HYP or a
guest) will safely fail, as donation requires the host PTE to be
either valid (host-owned) or completely empty.

Reviewed-by: Fuad Tabba <tabba@google.com>

Cheers,
/fuad

> + WARN_ON(host_stage2_try(kvm_pgtable_stage2_set_owner, &host_mmu.pgt, phys,
> + PAGE_SIZE, &host_s2_pool, PKVM_ID_HYP));
> +unlock:
> + hyp_unlock_component();
> + host_unlock_component();
> +
> + return ret;
> +}
> +
> +int __pkvm_hyp_donate_host_mmio(u64 pfn)
> +{
> + u64 phys = hyp_pfn_to_phys(pfn);
> + u64 virt = (u64)__hyp_va(phys);
> + size_t size = PAGE_SIZE;
> + int ret;
> + kvm_pte_t pte;
> +
> + if (addr_is_memory(phys))
> + return -EINVAL;
> +
> + host_lock_component();
> + hyp_lock_component();
> +
> + ret = kvm_pgtable_get_leaf(&pkvm_pgtable, (u64)virt, &pte, NULL);
> + if (ret)
> + goto unlock;
> + if (!kvm_pte_valid(pte)) {
> + ret = -ENOENT;
> + goto unlock;
> + }
> +
> + ret = kvm_pgtable_get_leaf(&host_mmu.pgt, phys, &pte, NULL);
> + if (ret)
> + goto unlock;
> +
> + if (FIELD_GET(KVM_INVALID_PTE_OWNER_MASK, pte) != PKVM_ID_HYP) {
> + ret = -EPERM;
> + goto unlock;
> + }
> +
> + WARN_ON(kvm_pgtable_hyp_unmap(&pkvm_pgtable, virt, size) != size);
> + WARN_ON(host_stage2_try(kvm_pgtable_stage2_set_owner, &host_mmu.pgt, phys,
> + PAGE_SIZE, &host_s2_pool, PKVM_ID_HOST));
> +unlock:
> + hyp_unlock_component();
> + host_unlock_component();
> +
> + return ret;
> +}
> +
> int __pkvm_host_donate_hyp(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 9b480f947da2..d954058e63ff 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1152,13 +1152,8 @@ 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);
> - }
> - return 0;
> - }
> + if (!kvm_pte_valid(ctx->old))
> + return stage2_pte_is_counted(ctx->old) ? -EPERM : 0;
>
> if (kvm_pte_table(ctx->old, ctx->level)) {
> childp = kvm_pte_follow(ctx->old, mm_ops);
> --
> 2.53.0.473.g4a7958ca14-goog
>