Re: [PATCH v3 2/2] LoongArch: Add paravirt support with vcpu_is_preempted() in guest side

[Bibo Mao]

On 2025/12/6 下午9:04, Huacai Chen wrote:

Hi, Bibo,

On Tue, Dec 2, 2025 at 10:48 AM Bibo Mao <maob...@loongson.cn> wrote:

Function vcpu_is_preempted() is used to check whether vCPU is preempted
or not. Here add implementation with vcpu_is_preempted() when option
CONFIG_PARAVIRT is enabled.

Signed-off-by: Bibo Mao <maob...@loongson.cn>
---
  arch/loongarch/include/asm/qspinlock.h |  3 +++
  arch/loongarch/kernel/paravirt.c       | 23 ++++++++++++++++++++++-
  2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/include/asm/qspinlock.h 
b/arch/loongarch/include/asm/qspinlock.h
index e76d3aa1e1eb..fa3eaf7e48f2 100644
--- a/arch/loongarch/include/asm/qspinlock.h
+++ b/arch/loongarch/include/asm/qspinlock.h
@@ -34,6 +34,9 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
         return true;
  }

+#define vcpu_is_preempted      vcpu_is_preempted
+bool vcpu_is_preempted(int cpu);
+
  #endif /* CONFIG_PARAVIRT */

  #include <asm-generic/qspinlock.h>
diff --git a/arch/loongarch/kernel/paravirt.c b/arch/loongarch/kernel/paravirt.c
index b1b51f920b23..b61a93c6aec8 100644
--- a/arch/loongarch/kernel/paravirt.c
+++ b/arch/loongarch/kernel/paravirt.c
@@ -246,6 +246,7 @@ static void pv_disable_steal_time(void)
  }

  #ifdef CONFIG_SMP
+static DEFINE_STATIC_KEY_FALSE(virt_preempt_key);
  static int pv_time_cpu_online(unsigned int cpu)
  {
         unsigned long flags;
@@ -267,6 +268,18 @@ static int pv_time_cpu_down_prepare(unsigned int cpu)

         return 0;
  }
+
+bool notrace vcpu_is_preempted(int cpu)
+{
+       struct kvm_steal_time *src;
+
+       if (!static_branch_unlikely(&virt_preempt_key))
+               return false;
+
+       src = &per_cpu(steal_time, cpu);
+       return !!(src->preempted & KVM_VCPU_PREEMPTED);
+}
+EXPORT_SYMBOL(vcpu_is_preempted);
  #endif

  static void pv_cpu_reboot(void *unused)
@@ -308,6 +321,9 @@ int __init pv_time_init(void)
                 pr_err("Failed to install cpu hotplug callbacks\n");
                 return r;
         }
+
+       if (kvm_para_has_feature(KVM_FEATURE_PREEMPT))
+               static_branch_enable(&virt_preempt_key);
  #endif

         static_call_update(pv_steal_clock, paravt_steal_clock);
@@ -318,7 +334,12 @@ int __init pv_time_init(void)
                 static_key_slow_inc(&paravirt_steal_rq_enabled);
  #endif

-       pr_info("Using paravirt steal-time\n");
+#ifdef CONFIG_SMP

Linux kernel is removing non-SMP step by step [1].
https://kernelnewbies.org/Linux_6.17#Unconditionally_compile_task_scheduler_with_SMP_support

Though we cannot remove all "#ifdef CONFIG_SMP" at present, we can at
least stop adding more.

So I prefer to make this whole patch out of CONFIG_SMP. But if you
don't like this, you can at least move the virt_preempt_key
declaration out of "#ifdef CONFIG_SMP", then the #ifdefs here can be
removed.

Sorry, I just notice this mail.
Will move virt_preempt_key out of CONFIG_SMP in next version.

Regards
Bibo Mao

Huacai

+       if (static_key_enabled(&virt_preempt_key))
+               pr_info("Using paravirt steal-time with preempt enabled\n");
+       else
+#endif
+               pr_info("Using paravirt steal-time with preempt disabled\n");

         return 0;
  }
--
2.39.3