[PATCH v12 22/23] KVM: selftests: Add ucall support for TDX

Sagi Shahar
2025-10-28T21:21:16Z

From: Ackerley Tng <ackerleytng@google.com>

ucalls for non-Coco VMs work by having the guest write to the rdi
register, then perform an io instruction to exit to the host. The host
then reads rdi using kvm_get_regs().

CPU registers can't be read using kvm_get_regs() for TDX, so TDX
guests use MMIO to pass the struct ucall's hva to the host. MMIO was
chosen because it is one of the simplest (hence unlikely to fail)
mechanisms that support passing 8 bytes from guest to host.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Co-developed-by: Sagi Shahar <sagis@google.com>
Signed-off-by: Sagi Shahar <sagis@google.com>

----------------------------------------------

Changes from v10:
* Removed ucall_arch_init() decleration from ucall.h.
* Replace vm_type type check with is_tdx_vm().
* Move mmio info initialization under is_tdx_vm() case.
---
.../selftests/kvm/include/ucall_common.h | 1 +
.../testing/selftests/kvm/include/x86/ucall.h | 6 ---
tools/testing/selftests/kvm/lib/x86/ucall.c | 46 +++++++++++++++++--
3 files changed, 42 insertions(+), 11 deletions(-)

diff --git a/tools/testing/selftests/kvm/include/ucall_common.h b/tools/testing/selftests/kvm/include/ucall_common.h
index d9d6581b8d4f..f5eebf690033 100644
--- a/tools/testing/selftests/kvm/include/ucall_common.h
+++ b/tools/testing/selftests/kvm/include/ucall_common.h
@@ -4,6 +4,7 @@
*/
#ifndef SELFTEST_KVM_UCALL_COMMON_H
#define SELFTEST_KVM_UCALL_COMMON_H
+#include "kvm_util.h"
#include "test_util.h"
#include "ucall.h"

diff --git a/tools/testing/selftests/kvm/include/x86/ucall.h b/tools/testing/selftests/kvm/include/x86/ucall.h
index d3825dcc3cd9..7e54ec2c1a45 100644
--- a/tools/testing/selftests/kvm/include/x86/ucall.h
+++ b/tools/testing/selftests/kvm/include/x86/ucall.h
@@ -2,12 +2,6 @@
#ifndef SELFTEST_KVM_UCALL_H
#define SELFTEST_KVM_UCALL_H

-#include "kvm_util.h"
-
#define UCALL_EXIT_REASON KVM_EXIT_IO

-static inline void ucall_arch_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa)
-{
-}
-
#endif
diff --git a/tools/testing/selftests/kvm/lib/x86/ucall.c b/tools/testing/selftests/kvm/lib/x86/ucall.c
index 1265cecc7dd1..fae6f37b0bcd 100644
--- a/tools/testing/selftests/kvm/lib/x86/ucall.c
+++ b/tools/testing/selftests/kvm/lib/x86/ucall.c
@@ -5,11 +5,35 @@
* Copyright (C) 2018, Red Hat, Inc.
*/
#include "kvm_util.h"
+#include "tdx/tdx.h"
+#include "tdx/tdx_util.h"

#define UCALL_PIO_PORT ((uint16_t)0x1000)

+static uint8_t vm_type;
+static vm_paddr_t host_ucall_mmio_gpa;
+static vm_paddr_t ucall_mmio_gpa;
+
+void ucall_arch_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa)
+{
+ vm_type = vm->type;
+ sync_global_to_guest(vm, vm_type);
+
+ if (is_tdx_vm(vm)) {
+ host_ucall_mmio_gpa = ucall_mmio_gpa = mmio_gpa;
+ ucall_mmio_gpa |= vm->arch.s_bit;
+ }
+
+ sync_global_to_guest(vm, ucall_mmio_gpa);
+}
+
void ucall_arch_do_ucall(vm_vaddr_t uc)
{
+ if (vm_type == KVM_X86_TDX_VM) {
+ tdg_vp_vmcall_ve_request_mmio_write(ucall_mmio_gpa, 8, uc);
+ return;
+ }
+
/*
* FIXME: Revert this hack (the entire commit that added it) once nVMX
* preserves L2 GPRs across a nested VM-Exit. If a ucall from L2, e.g.
@@ -46,11 +70,23 @@ void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu)
{
struct kvm_run *run = vcpu->run;

- if (run->exit_reason == KVM_EXIT_IO && run->io.port == UCALL_PIO_PORT) {
- struct kvm_regs regs;
+ switch (vm_type) {
+ case KVM_X86_TDX_VM:
+ if (vcpu->run->exit_reason == KVM_EXIT_MMIO &&
+ vcpu->run->mmio.phys_addr == host_ucall_mmio_gpa &&
+ vcpu->run->mmio.len == 8 && vcpu->run->mmio.is_write) {
+ uint64_t data = *(uint64_t *)vcpu->run->mmio.data;
+
+ return (void *)data;
+ }
+ return NULL;
+ default:
+ if (run->exit_reason == KVM_EXIT_IO && run->io.port == UCALL_PIO_PORT) {
+ struct kvm_regs regs;

- vcpu_regs_get(vcpu, &regs);
- return (void *)regs.rdi;
+ vcpu_regs_get(vcpu, &regs);
+ return (void *)regs.rdi;
+ }
+ return NULL;
}
- return NULL;
}
--
2.51.1.851.g4ebd6896fd-goog