Re: [PATCH v9 18/19] KVM: selftests: Add ucall support for TDX

[Binbin Wu]

On 8/21/2025 12:29 PM, Sagi Shahar wrote:

From: Ackerley Tng <ackerley...@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 <ackerley...@google.com>
Co-developed-by: Sagi Shahar <sa...@google.com>
Signed-off-by: Sagi Shahar <sa...@google.com>
---
  .../testing/selftests/kvm/include/x86/ucall.h |  4 +-
  tools/testing/selftests/kvm/lib/x86/ucall.c   | 45 ++++++++++++++++---
  2 files changed, 41 insertions(+), 8 deletions(-)

diff --git a/tools/testing/selftests/kvm/include/x86/ucall.h 
b/tools/testing/selftests/kvm/include/x86/ucall.h
index d3825dcc3cd9..0494a4a21557 100644
--- a/tools/testing/selftests/kvm/include/x86/ucall.h
+++ b/tools/testing/selftests/kvm/include/x86/ucall.h
@@ -6,8 +6,6 @@

#define UCALL_EXIT_REASON KVM_EXIT_IO -static inline void ucall_arch_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa)

-{
-}
+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..0ad24baaa3c4 100644
--- a/tools/testing/selftests/kvm/lib/x86/ucall.c
+++ b/tools/testing/selftests/kvm/lib/x86/ucall.c
@@ -5,11 +5,34 @@
   * Copyright (C) 2018, Red Hat, Inc.
   */
  #include "kvm_util.h"
+#include "tdx/tdx.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);
+
+       host_ucall_mmio_gpa = ucall_mmio_gpa = mmio_gpa;
+
+       if (vm_type == KVM_X86_TDX_VM)
+               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 +69,23 @@ 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;

My first thought was how did SEV_ES or SNP work for this since they are not
able to get RDI neither.
Then I had a check in sev_smoke_test.c, both guest_sev_es_code() and
guest_snp_code() call GUEST_ASSERT(), which finally calls ucall_assert(), but
in test_sev(), the code doesn't handle ucall for SEV_ES or SNP.
Does it mean GUEST_ASSERT() is currently not working and ignored for SEV_ES
and SNP? Or did I miss anything?

+       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;
  }