Re: [PATCH v2 1/3] target/loongarch/kvm: advertise pv features per vCPU

[Bibo Mao]

On 2026/7/10 上午8:50, Tao Cui wrote:
From: Tao Cui <cui...@kylinos.cn>

kvm_set_pv_features() programs the KVM_FEATURE cpucfg attribute, which is a
per-vCPU setting. It was called from kvm_arch_put_registers() under a
function-local static guard, so it ran only once for the whole VM: only the
first vCPU got its pv features pushed to KVM, and on SMP guests the others
never saw KVM_FEATURE_IPI / KVM_FEATURE_STEAL_TIME.

Drop the static guard and push pv features per vCPU under
KVM_PUT_FULL_STATE, the same gate kvm_set_stealtime() already uses. Host
feature detection stays in kvm_arch_init_vcpu(); the per-vCPU state write
belongs in kvm_arch_put_registers().

Signed-off-by: Tao Cui <cui...@kylinos.cn>
---
  target/loongarch/kvm/kvm.c | 15 ++++++---------
  1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/target/loongarch/kvm/kvm.c b/target/loongarch/kvm/kvm.c
index d6539c12ac..c557ee3c3d 100644
--- a/target/loongarch/kvm/kvm.c
+++ b/target/loongarch/kvm/kvm.c
@@ -816,7 +816,6 @@ int kvm_arch_get_registers(CPUState *cs, Error **errp)
  int kvm_arch_put_registers(CPUState *cs, KvmPutState level, Error **errp)
  {
      int ret;
-    static int once;
  
      ret = kvm_loongarch_put_regs_core(cs);
      if (ret) {
@@ -843,19 +842,17 @@ int kvm_arch_put_registers(CPUState *cs, KvmPutState 
level, Error **errp)
          return ret;
      }
  
-    if (!once) {
+    if (level >= KVM_PUT_FULL_STATE) {
+        /*
+         * pv_features and steal time are per-vCPU state. Push them on
+         * full-state sync so every vCPU gets its own settings; the kernel
+         * clears the steal-time guest_addr on KVM_PUT_RESET_STATE.
+         */
          ret = kvm_set_pv_features(cs);
pv feature is a little different from steal-time. steal-time guest_addr 
is created from guest OS, pv feature is created from VMM at beginning. 
steal-time guest_addr can be set for many times, and there is bit 
KVM_STEAL_PHYS_VALID checking with steal-time guest_addr, however pv 
feature can be set only once with existing method.

Although I do not understand flow of VM migration, with 
KVM_PUT_FULL_STATE state changing, there are at least two places where 
this state is set, one is from cpu_common_realizefn() which calls 
cpu_synchronize_post_init(), the other is 
qemu_loadvm_state()/qemu_loadvm_state_main() which calls 
cpu_synchronize_all_post_init().

It seems that VM will fail to migrate since kvm_set_pv_features is 
called twice at least here. Do you test VM migration with this patch?

Regards
Bibo Mao
          if (ret) {
              return ret;
          }
-        once = 1;
-    }
  
-    if (level >= KVM_PUT_FULL_STATE) {
-        /*
-         * only KVM_PUT_FULL_STATE is required, kvm kernel will clear
-         * guest_addr for KVM_PUT_RESET_STATE
-         */
          ret = kvm_set_stealtime(cs);
          if (ret) {
              return ret;