linux-kernel - Re: [PATCH v2 2/3] LoongArch: Add paravirt support with vcpu_is

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Message-ID: <5d80c452-bbc3-539a-fb8d-14dbe353f8cb@loongson.cn>
Date: Mon, 24 Nov 2025 15:00:40 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, WANG Xuerui <kernel@...0n.name>,
 Juergen Gross <jgross@...e.com>, Ajay Kaher <ajay.kaher@...adcom.com>,
 Alexey Makhalov <alexey.makhalov@...adcom.com>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>, kvm@...r.kernel.org,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux.dev, x86@...nel.org
Subject: Re: [PATCH v2 2/3] LoongArch: Add paravirt support with
 vcpu_is_preempted() in guest side



On 2025/11/24 下午2:33, Huacai Chen wrote:
> Hi, Bibo,
> 
> On Mon, Nov 24, 2025 at 11:54 AM Bibo Mao <maobibo@...ngson.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 <maobibo@...ngson.cn>
>> ---
>>   arch/loongarch/include/asm/qspinlock.h |  5 +++++
>>   arch/loongarch/kernel/paravirt.c       | 16 ++++++++++++++++
>>   2 files changed, 21 insertions(+)
>>
>> diff --git a/arch/loongarch/include/asm/qspinlock.h b/arch/loongarch/include/asm/qspinlock.h
>> index e76d3aa1e1eb..9a5b7ba1f4cb 100644
>> --- a/arch/loongarch/include/asm/qspinlock.h
>> +++ b/arch/loongarch/include/asm/qspinlock.h
>> @@ -34,6 +34,11 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
>>          return true;
>>   }
>>
>> +#ifdef CONFIG_SMP
>> +#define vcpu_is_preempted      vcpu_is_preempted
>> +bool vcpu_is_preempted(int cpu);
> In V1 there is a build error because you reference mp_ops, so in V2
> you needn't put it in CONFIG_SMP.
The compile failure problem is that vcpu_is_preempted() is redefined in 
both arch/loongarch/kernel/paravirt.c and include/linux/sched.h

The problem is that <asm/spinlock.h> is not included by sched.h, if 
CONFIG_SMP is disabled. Here is part of file include/linux/spinlock.h
#ifdef CONFIG_SMP
# include <asm/spinlock.h>
#else
# include <linux/spinlock_up.h>
#endif

> On the other hand, even if you really build a UP guest kernel, when
> multiple guests run together, you probably need vcpu_is_preemtped.
It is not relative with multiple VMs. When vcpu_is_preempted() is 
called, it is to detect whether dest CPU is preempted or not, the cpu 
from smp_processor_id() should not be preempted. So in generic 
vcpu_is_preempted() works on multiple vCPUs.

Regards
Bibo Mao
> 
> 
> Huacai
> 
>> +#endif
>> +
>>   #endif /* CONFIG_PARAVIRT */
>>
>>   #include <asm-generic/qspinlock.h>
>> diff --git a/arch/loongarch/kernel/paravirt.c b/arch/loongarch/kernel/paravirt.c
>> index b1b51f920b23..d4163679adc4 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
>> +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);
>> --
>> 2.39.3
>>