Re: [PATCH v3 14/20] i386/kvm: Add save/load support for KVM_REG_GUEST_SSP

[Xiaoyao Li]

On 10/24/2025 2:56 PM, Zhao Liu wrote:

From: Yang Weijiang <weijiang.y...@intel.com>

CET provides a new architectural register, shadow stack pointer (SSP),
which cannot be directly encoded as a source, destination or memory
operand in instructions. But Intel VMCS & VMCB provide fields to
save/load guest & host's ssp.

It's necessary to save & load Guest's ssp before & after migration. To
support this, KVM implements Guest's SSP as a special KVM internal
register - KVM_REG_GUEST_SSP, and allows QEMU to save & load it via
KVM_GET_ONE_REG/KVM_SET_ONE_REG.

Cache KVM_REG_GUEST_SSP in X86CPUState.

Tested-by: Farrah Chen <farrah.c...@intel.com>
Signed-off-by: Yang Weijiang <weijiang.y...@intel.com>
Co-developed-by: Chao Gao <chao....@intel.com>
Signed-off-by: Chao Gao <chao....@intel.com>
Co-developed-by: Zhao Liu <zhao1....@intel.com>
Signed-off-by: Zhao Liu <zhao1....@intel.com>

Reviewed-by: Xiaoyao Li <xiaoyao...@intel.com>

---
  target/i386/cpu.h     |  1 +
  target/i386/kvm/kvm.c | 39 +++++++++++++++++++++++++++++++++++++++
  2 files changed, 40 insertions(+)

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 4edb977575e2..ad4287822831 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -2105,6 +2105,7 @@ typedef struct CPUArchState {
      uint64_t pl2_ssp;
      uint64_t pl3_ssp;
      uint64_t int_ssp_table;
+    uint64_t guest_ssp;

/* Fields up to this point are cleared by a CPU reset */

      struct {} end_reset_fields;
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 92c2fd6d6aee..412e99ba5b53 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -4280,6 +4280,35 @@ static int kvm_put_msrs(X86CPU *cpu, KvmPutState level)
      return kvm_buf_set_msrs(cpu);
  }

+static int kvm_put_kvm_regs(X86CPU *cpu)

+{
+    CPUX86State *env = &cpu->env;
+    int ret;
+
+    if ((env->features[FEAT_7_0_ECX] & CPUID_7_0_ECX_CET_SHSTK)) {
+        ret = kvm_set_one_reg(CPU(cpu), KVM_X86_REG_KVM(KVM_REG_GUEST_SSP),
+                              &env->guest_ssp);
+        if (ret) {
+            return ret;
+        }
+    }
+    return 0;
+}
+
+static int kvm_get_kvm_regs(X86CPU *cpu)
+{
+    CPUX86State *env = &cpu->env;
+    int ret;
+
+    if ((env->features[FEAT_7_0_ECX] & CPUID_7_0_ECX_CET_SHSTK)) {
+        ret = kvm_get_one_reg(CPU(cpu), KVM_X86_REG_KVM(KVM_REG_GUEST_SSP),
+                              &env->guest_ssp);
+        if (ret) {
+            return ret;
+        }
+    }
+    return 0;
+}

static int kvm_get_xsave(X86CPU *cpu)

  {
@@ -5425,6 +5454,11 @@ int kvm_arch_put_registers(CPUState *cpu, KvmPutState 
level, Error **errp)
          error_setg_errno(errp, -ret, "Failed to set MSRs");
          return ret;
      }
+    ret = kvm_put_kvm_regs(x86_cpu);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret, "Failed to set KVM type registers");
+        return ret;
+    }
      ret = kvm_put_vcpu_events(x86_cpu, level);
      if (ret < 0) {
          error_setg_errno(errp, -ret, "Failed to set vCPU events");
@@ -5497,6 +5531,11 @@ int kvm_arch_get_registers(CPUState *cs, Error **errp)
          error_setg_errno(errp, -ret, "Failed to get MSRs");
          goto out;
      }
+    ret = kvm_get_kvm_regs(cpu);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret, "Failed to get KVM type registers");
+        goto out;
+    }
      ret = kvm_get_apic(cpu);
      if (ret < 0) {
          error_setg_errno(errp, -ret, "Failed to get APIC");