[PATCH 24/30] KVM: arm64: Implement the MEM_SHARE hypercall for protected VMs

Will Deacon
2026-01-05T15:51:05Z

Implement the ARM_SMCCC_KVM_FUNC_MEM_SHARE hypercall to allow protected
VMs to share memory (e.g. the swiotlb bounce buffers) back to the host.

Signed-off-by: Will Deacon <will@kernel.org>
---
arch/arm64/kvm/hyp/include/nvhe/mem_protect.h | 1 +
arch/arm64/kvm/hyp/nvhe/mem_protect.c | 32 ++++++++++
arch/arm64/kvm/hyp/nvhe/pkvm.c | 61 +++++++++++++++++++
3 files changed, 94 insertions(+)

diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
index 5e6cdafcdd69..42fd60c5cfc9 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
@@ -35,6 +35,7 @@ extern unsigned long hyp_nr_cpus;

int __pkvm_prot_finalize(void);
int __pkvm_host_share_hyp(u64 pfn);
+int __pkvm_guest_share_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);
int __pkvm_hyp_donate_host(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 57bc7ade30de..365c769c82a4 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -888,6 +888,38 @@ int __pkvm_host_share_hyp(u64 pfn)
return ret;
}

+int __pkvm_guest_share_host(struct pkvm_hyp_vcpu *vcpu, u64 gfn)
+{
+ struct pkvm_hyp_vm *vm = pkvm_hyp_vcpu_to_hyp_vm(vcpu);
+ u64 phys, ipa = hyp_pfn_to_phys(gfn);
+ kvm_pte_t pte;
+ int ret;
+
+ host_lock_component();
+ guest_lock_component(vm);
+
+ ret = get_valid_guest_pte(vm, ipa, &pte, &phys);
+ if (ret)
+ goto unlock;
+
+ ret = -EPERM;
+ if (pkvm_getstate(kvm_pgtable_stage2_pte_prot(pte)) != PKVM_PAGE_OWNED)
+ goto unlock;
+ if (__host_check_page_state_range(phys, PAGE_SIZE, PKVM_NOPAGE))
+ goto unlock;
+
+ ret = 0;
+ WARN_ON(kvm_pgtable_stage2_map(&vm->pgt, ipa, PAGE_SIZE, phys,
+ pkvm_mkstate(KVM_PGTABLE_PROT_RWX, PKVM_PAGE_SHARED_OWNED),
+ &vcpu->vcpu.arch.pkvm_memcache, 0));
+ WARN_ON(__host_set_page_state_range(phys, PAGE_SIZE, PKVM_PAGE_SHARED_BORROWED));
+unlock:
+ guest_unlock_component(vm);
+ host_unlock_component();
+
+ return ret;
+}
+
int __pkvm_host_unshare_hyp(u64 pfn)
{
u64 phys = hyp_pfn_to_phys(pfn);
diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index 5cdec49e989b..d8afa2b98542 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -936,6 +936,58 @@ int __pkvm_finalize_teardown_vm(pkvm_handle_t handle)
hyp_spin_unlock(&vm_table_lock);
return err;
}
+
+static u64 __pkvm_memshare_page_req(struct kvm_vcpu *vcpu, u64 ipa)
+{
+ u64 elr;
+
+ /* Fake up a data abort (level 3 translation fault on write) */
+ vcpu->arch.fault.esr_el2 = (ESR_ELx_EC_DABT_LOW << ESR_ELx_EC_SHIFT) |
+ ESR_ELx_WNR | ESR_ELx_FSC_FAULT |
+ FIELD_PREP(ESR_ELx_FSC_LEVEL, 3);
+
+ /* Shuffle the IPA around into the HPFAR */
+ vcpu->arch.fault.hpfar_el2 = (HPFAR_EL2_NS | (ipa >> 8)) & HPFAR_MASK;
+
+ /* This is a virtual address. 0's good. Let's go with 0. */
+ vcpu->arch.fault.far_el2 = 0;
+
+ /* Rewind the ELR so we return to the HVC once the IPA is mapped */
+ elr = read_sysreg(elr_el2);
+ elr -= 4;
+ write_sysreg(elr, elr_el2);
+
+ return ARM_EXCEPTION_TRAP;
+}
+
+static bool pkvm_memshare_call(u64 *ret, struct kvm_vcpu *vcpu, u64 *exit_code)
+{
+ struct pkvm_hyp_vcpu *hyp_vcpu;
+ u64 ipa = smccc_get_arg1(vcpu);
+
+ if (!PAGE_ALIGNED(ipa))
+ goto out_guest;
+
+ hyp_vcpu = container_of(vcpu, struct pkvm_hyp_vcpu, vcpu);
+ switch (__pkvm_guest_share_host(hyp_vcpu, hyp_phys_to_pfn(ipa))) {
+ case 0:
+ ret[0] = SMCCC_RET_SUCCESS;
+ goto out_guest;
+ case -ENOENT:
+ /*
+ * Convert the exception into a data abort so that the page
+ * being shared is mapped into the guest next time.
+ */
+ *exit_code = __pkvm_memshare_page_req(vcpu, ipa);
+ goto out_host;
+ }
+
+out_guest:
+ return true;
+out_host:
+ return false;
+}
+
/*
* Handler for protected VM HVC calls.
*
@@ -952,6 +1004,7 @@ bool kvm_handle_pvm_hvc64(struct kvm_vcpu *vcpu, u64 *exit_code)
case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
val[0] = BIT(ARM_SMCCC_KVM_FUNC_FEATURES);
val[0] |= BIT(ARM_SMCCC_KVM_FUNC_HYP_MEMINFO);
+ val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MEM_SHARE);
break;
case ARM_SMCCC_VENDOR_HYP_KVM_HYP_MEMINFO_FUNC_ID:
if (smccc_get_arg1(vcpu) ||
@@ -962,6 +1015,14 @@ bool kvm_handle_pvm_hvc64(struct kvm_vcpu *vcpu, u64 *exit_code)

val[0] = PAGE_SIZE;
break;
+ case ARM_SMCCC_VENDOR_HYP_KVM_MEM_SHARE_FUNC_ID:
+ if (smccc_get_arg2(vcpu) ||
+ smccc_get_arg3(vcpu)) {
+ break;
+ }
+
+ handled = pkvm_memshare_call(val, vcpu, exit_code);
+ break;
default:
/* Punt everything else back to the host, for now. */
handled = false;
--
2.52.0.351.gbe84eed79e-goog