[PATCH RFC v1 2/2] i386: reorder kvm_put_sregs2() and kvm_put_nested_state() when vCPU is reset

[Vitaly Kuznetsov]

Setting nested state upon migration needs to happen after kvm_put_sregs2()
to e.g. have EFER.SVME set. This, however, doesn't work for vCPU reset:
when vCPU is in VMX root operation, certain CR bits are locked and
kvm_put_sregs2() may fail. As nested state is fully cleaned up upon
vCPU reset (kvm_arch_reset_vcpu() -> kvm_init_nested_state()), calling
kvm_put_nested_state() before kvm_put_sregs2() is OK, this will ensure
that vCPU is *not* in VMX root opertaion.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 target/i386/kvm/kvm.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 4f8dacc1d4b5..73e3880fa57b 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -4529,18 +4529,34 @@ int kvm_arch_put_registers(CPUState *cpu, int level)
 
     assert(cpu_is_stopped(cpu) || qemu_cpu_is_self(cpu));
 
-    /* must be before kvm_put_nested_state so that EFER.SVME is set */
+    /*
+     * When resetting a vCPU, make sure to reset nested state first to
+     * e.g clear VMXON state and unlock certain CR4 bits.
+     */
+    if (level == KVM_PUT_RESET_STATE) {
+        ret = kvm_put_nested_state(x86_cpu);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
     ret = has_sregs2 ? kvm_put_sregs2(x86_cpu) : kvm_put_sregs(x86_cpu);
     if (ret < 0) {
         return ret;
     }
 
-    if (level >= KVM_PUT_RESET_STATE) {
+    /*
+     * When putting full CPU state, kvm_put_nested_state() must happen after
+     * kvm_put_sregs{,2} so that e.g. EFER.SVME is already set.
+     */
+    if (level == KVM_PUT_FULL_STATE) {
         ret = kvm_put_nested_state(x86_cpu);
         if (ret < 0) {
             return ret;
         }
+    }
 
+    if (level >= KVM_PUT_RESET_STATE) {
         ret = kvm_put_msr_feature_control(x86_cpu);
         if (ret < 0) {
             return ret;
-- 
2.37.1