Re: [PATCH v2 24/35] KVM: arm64: Introduce hypercall to force reclaim of a protected page

Alexandru Elisei
2026-02-12T17:18:57Z

Hi Will,

On Mon, Jan 19, 2026 at 12:46:17PM +0000, Will Deacon wrote:
> Introduce a new hypercall, __pkvm_force_reclaim_guest_page(), to allow
> the host to forcefully reclaim a physical page that was previous donated
> to a protected guest. This results in the page being zeroed and the
> previous guest mapping being poisoned so that new pages cannot be
> subsequently donated at the same IPA.
>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> arch/arm64/include/asm/kvm_asm.h | 1 +
> arch/arm64/include/asm/kvm_pgtable.h | 6 +
> arch/arm64/kvm/hyp/include/nvhe/mem_protect.h | 1 +
> arch/arm64/kvm/hyp/include/nvhe/memory.h | 6 +
> arch/arm64/kvm/hyp/include/nvhe/pkvm.h | 1 +
> arch/arm64/kvm/hyp/nvhe/hyp-main.c | 8 ++
> arch/arm64/kvm/hyp/nvhe/mem_protect.c | 127 +++++++++++++++++-
> arch/arm64/kvm/hyp/nvhe/pkvm.c | 4 +-
> 8 files changed, 152 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index 2e7e8e7771f6..39e4e588ca4f 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -90,6 +90,7 @@ enum __kvm_host_smccc_func {
> __KVM_HOST_SMCCC_FUNC___pkvm_unreserve_vm,
> __KVM_HOST_SMCCC_FUNC___pkvm_init_vm,
> __KVM_HOST_SMCCC_FUNC___pkvm_init_vcpu,
> + __KVM_HOST_SMCCC_FUNC___pkvm_force_reclaim_guest_page,
> __KVM_HOST_SMCCC_FUNC___pkvm_reclaim_dying_guest_page,
> __KVM_HOST_SMCCC_FUNC___pkvm_start_teardown_vm,
> __KVM_HOST_SMCCC_FUNC___pkvm_finalize_teardown_vm,
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index eb2a6258d83d..4c069f875a85 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -114,6 +114,12 @@ enum kvm_invalid_pte_type {
> * ownership.
> */
> KVM_HOST_INVALID_PTE_TYPE_DONATION,
> +
> + /*
> + * The page has been forcefully reclaimed from the guest by the
> + * host.
> + */
> + KVM_GUEST_INVALID_PTE_TYPE_POISONED,
> };
>
> static inline bool kvm_pte_valid(kvm_pte_t pte)
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> index cde38a556049..f27b037abaf3 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> @@ -41,6 +41,7 @@ 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);
> int __pkvm_host_donate_guest(u64 pfn, u64 gfn, struct pkvm_hyp_vcpu *vcpu);
> +int __pkvm_host_force_reclaim_page_guest(phys_addr_t phys);
> int __pkvm_host_reclaim_page_guest(u64 gfn, struct pkvm_hyp_vm *vm);
> int __pkvm_host_share_guest(u64 pfn, u64 gfn, u64 nr_pages, struct pkvm_hyp_vcpu *vcpu,
> enum kvm_pgtable_prot prot);
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h
> index dee1a406b0c2..4cedb720c75d 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/memory.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/memory.h
> @@ -30,6 +30,12 @@ enum pkvm_page_state {
> * struct hyp_page.
> */
> PKVM_NOPAGE = BIT(0) | BIT(1),
> +
> + /*
> + * 'Meta-states' which aren't encoded directly in the PTE's SW bits (or
> + * the hyp_vmemmap entry for the host)
> + */
> + PKVM_POISON = BIT(2),
> };
> #define PKVM_PAGE_STATE_MASK (BIT(0) | BIT(1))

Looks a bit awkward to me, having the page state encoded using 3 bits, but the
mask only 2 bits.

Thanks,
Alex

>
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/pkvm.h b/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
> index 506831804f64..a5a7bb453f3e 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
> @@ -78,6 +78,7 @@ int __pkvm_reclaim_dying_guest_page(pkvm_handle_t handle, u64 gfn);
> int __pkvm_start_teardown_vm(pkvm_handle_t handle);
> int __pkvm_finalize_teardown_vm(pkvm_handle_t handle);
>
> +struct pkvm_hyp_vm *get_vm_by_handle(pkvm_handle_t handle);
> struct pkvm_hyp_vcpu *pkvm_load_hyp_vcpu(pkvm_handle_t handle,
> unsigned int vcpu_idx);
> void pkvm_put_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu);
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index f43c50ae2d81..e68b5d24bdad 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -570,6 +570,13 @@ static void handle___pkvm_init_vcpu(struct kvm_cpu_context *host_ctxt)
> cpu_reg(host_ctxt, 1) = __pkvm_init_vcpu(handle, host_vcpu, vcpu_hva);
> }
>
> +static void handle___pkvm_force_reclaim_guest_page(struct kvm_cpu_context *host_ctxt)
> +{
> + DECLARE_REG(phys_addr_t, phys, host_ctxt, 1);
> +
> + cpu_reg(host_ctxt, 1) = __pkvm_host_force_reclaim_page_guest(phys);
> +}
> +
> static void handle___pkvm_reclaim_dying_guest_page(struct kvm_cpu_context *host_ctxt)
> {
> DECLARE_REG(pkvm_handle_t, handle, host_ctxt, 1);
> @@ -631,6 +638,7 @@ static const hcall_t host_hcall[] = {
> HANDLE_FUNC(__pkvm_unreserve_vm),
> HANDLE_FUNC(__pkvm_init_vm),
> HANDLE_FUNC(__pkvm_init_vcpu),
> + HANDLE_FUNC(__pkvm_force_reclaim_guest_page),
> HANDLE_FUNC(__pkvm_reclaim_dying_guest_page),
> HANDLE_FUNC(__pkvm_start_teardown_vm),
> HANDLE_FUNC(__pkvm_finalize_teardown_vm),
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index f4638fe9d77a..49b309b8d7d2 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -613,6 +613,35 @@ static u64 host_stage2_encode_gfn_meta(struct pkvm_hyp_vm *vm, u64 gfn)
> FIELD_PREP(KVM_HOST_PTE_OWNER_GUEST_GFN_MASK, gfn);
> }
>
> +static int host_stage2_decode_gfn_meta(kvm_pte_t pte, struct pkvm_hyp_vm **vm,
> + u64 *gfn)
> +{
> + pkvm_handle_t handle;
> + u64 meta;
> +
> + if (WARN_ON(kvm_pte_valid(pte)))
> + return -EINVAL;
> +
> + if (FIELD_GET(KVM_INVALID_PTE_TYPE_MASK, pte) !=
> + KVM_HOST_INVALID_PTE_TYPE_DONATION) {
> + return -EINVAL;
> + }
> +
> + if (FIELD_GET(KVM_HOST_DONATION_PTE_OWNER_MASK, pte) != PKVM_ID_GUEST)
> + return -EPERM;
> +
> + meta = FIELD_GET(KVM_HOST_DONATION_PTE_EXTRA_MASK, pte);
> + handle = FIELD_GET(KVM_HOST_PTE_OWNER_GUEST_HANDLE_MASK, meta);
> + *vm = get_vm_by_handle(handle);
> + if (!*vm) {
> + /* We probably raced with teardown; try again */
> + return -EAGAIN;
> + }
> +
> + *gfn = FIELD_GET(KVM_HOST_PTE_OWNER_GUEST_GFN_MASK, meta);
> + return 0;
> +}
> +
> static bool host_stage2_force_pte_cb(u64 addr, u64 end, enum kvm_pgtable_prot prot)
> {
> /*
> @@ -809,8 +838,20 @@ static int __hyp_check_page_state_range(phys_addr_t phys, u64 size, enum pkvm_pa
> return 0;
> }
>
> +static bool guest_pte_is_poisoned(kvm_pte_t pte)
> +{
> + if (kvm_pte_valid(pte))
> + return false;
> +
> + return FIELD_GET(KVM_INVALID_PTE_TYPE_MASK, pte) ==
> + KVM_GUEST_INVALID_PTE_TYPE_POISONED;
> +}
> +
> static enum pkvm_page_state guest_get_page_state(kvm_pte_t pte, u64 addr)
> {
> + if (guest_pte_is_poisoned(pte))
> + return PKVM_POISON;
> +
> if (!kvm_pte_valid(pte))
> return PKVM_NOPAGE;
>
> @@ -839,6 +880,8 @@ static int get_valid_guest_pte(struct pkvm_hyp_vm *vm, u64 ipa, kvm_pte_t *ptep,
> ret = kvm_pgtable_get_leaf(&vm->pgt, ipa, &pte, &level);
> if (ret)
> return ret;
> + if (guest_pte_is_poisoned(pte))
> + return -EHWPOISON;
> if (!kvm_pte_valid(pte))
> return -ENOENT;
> if (level != KVM_PGTABLE_LAST_LEVEL)
> @@ -1104,6 +1147,84 @@ static void hyp_poison_page(phys_addr_t phys)
> hyp_fixmap_unmap();
> }
>
> +static int host_stage2_get_guest_info(phys_addr_t phys, struct pkvm_hyp_vm **vm,
> + u64 *gfn)
> +{
> + enum pkvm_page_state state;
> + kvm_pte_t pte;
> + s8 level;
> + int ret;
> +
> + if (!addr_is_memory(phys))
> + return -EFAULT;
> +
> + state = get_host_state(hyp_phys_to_page(phys));
> + switch (state) {
> + case PKVM_PAGE_OWNED:
> + case PKVM_PAGE_SHARED_OWNED:
> + case PKVM_PAGE_SHARED_BORROWED:
> + /* The access should no longer fault; try again. */
> + return -EAGAIN;
> + case PKVM_NOPAGE:
> + break;
> + default:
> + return -EPERM;
> + }
> +
> + ret = kvm_pgtable_get_leaf(&host_mmu.pgt, phys, &pte, &level);
> + if (ret)
> + return ret;
> +
> + if (WARN_ON(level != KVM_PGTABLE_LAST_LEVEL))
> + return -EINVAL;
> +
> + return host_stage2_decode_gfn_meta(pte, vm, gfn);
> +}
> +
> +int __pkvm_host_force_reclaim_page_guest(phys_addr_t phys)
> +{
> + struct pkvm_hyp_vm *vm;
> + u64 gfn, ipa, pa;
> + kvm_pte_t pte;
> + int ret;
> +
> + hyp_spin_lock(&vm_table_lock);
> + host_lock_component();
> +
> + ret = host_stage2_get_guest_info(phys, &vm, &gfn);
> + if (ret)
> + goto unlock_host;
> +
> + ipa = hyp_pfn_to_phys(gfn);
> + guest_lock_component(vm);
> + ret = get_valid_guest_pte(vm, ipa, &pte, &pa);
> + if (ret)
> + goto unlock_guest;
> +
> + WARN_ON(pa != phys);
> + if (guest_get_page_state(pte, ipa) != PKVM_PAGE_OWNED) {
> + ret = -EPERM;
> + goto unlock_guest;
> + }
> +
> + /* We really shouldn't be allocating, so don't pass a memcache */
> + ret = kvm_pgtable_stage2_annotate(&vm->pgt, ipa, PAGE_SIZE, NULL,
> + KVM_GUEST_INVALID_PTE_TYPE_POISONED,
> + 0);
> + if (ret)
> + goto unlock_guest;
> +
> + hyp_poison_page(phys);
> + WARN_ON(host_stage2_set_owner_locked(phys, PAGE_SIZE, PKVM_ID_HOST));
> +unlock_guest:
> + guest_unlock_component(vm);
> +unlock_host:
> + host_unlock_component();
> + hyp_spin_unlock(&vm_table_lock);
> +
> + return ret;
> +}
> +
> int __pkvm_host_reclaim_page_guest(u64 gfn, struct pkvm_hyp_vm *vm)
> {
> u64 ipa = hyp_pfn_to_phys(gfn);
> @@ -1138,7 +1259,11 @@ int __pkvm_host_reclaim_page_guest(u64 gfn, struct pkvm_hyp_vm *vm)
> guest_unlock_component(vm);
> host_unlock_component();
>
> - return ret;
> + /*
> + * -EHWPOISON implies that the page was forcefully reclaimed already
> + * so return success for the GUP pin to be dropped.
> + */
> + return ret && ret != -EHWPOISON ? ret : 0;
> }
>
> int __pkvm_host_donate_guest(u64 pfn, u64 gfn, struct pkvm_hyp_vcpu *vcpu)
> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> index c5772417372d..2836c68c1ea5 100644
> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> @@ -231,10 +231,12 @@ void pkvm_hyp_vm_table_init(void *tbl)
> /*
> * Return the hyp vm structure corresponding to the handle.
> */
> -static struct pkvm_hyp_vm *get_vm_by_handle(pkvm_handle_t handle)
> +struct pkvm_hyp_vm *get_vm_by_handle(pkvm_handle_t handle)
> {
> unsigned int idx = vm_handle_to_idx(handle);
>
> + hyp_assert_lock_held(&vm_table_lock);
> +
> if (unlikely(idx >= KVM_MAX_PVMS))
> return NULL;
>
> --
> 2.52.0.457.g6b5491de43-goog
>
>