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Message-ID: <c14346a5-37ec-0fef-f143-c18227fec635@loongson.cn>
Date: Wed, 19 Nov 2025 11:08:19 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, WANG Xuerui <kernel@...0n.name>,
Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
Waiman Long <longman@...hat.com>, 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 2/3] LoongArch: Add paravirt support with
vcpu_is_preempted()
On 2025/11/19 上午10:58, Huacai Chen wrote:
> On Wed, Nov 19, 2025 at 10:01 AM Bibo Mao <maobibo@...ngson.cn> wrote:
>>
>>
>>
>> On 2025/11/18 下午8:48, Huacai Chen wrote:
>>> Hi, Bibo,
>>>
>>> On Tue, Nov 18, 2025 at 4:07 PM 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/smp.h | 1 +
>>>> arch/loongarch/include/asm/spinlock.h | 5 +++++
>>>> arch/loongarch/kernel/paravirt.c | 16 ++++++++++++++++
>>>> arch/loongarch/kernel/smp.c | 6 ++++++
>>>> 4 files changed, 28 insertions(+)
>>>>
>>>> diff --git a/arch/loongarch/include/asm/smp.h b/arch/loongarch/include/asm/smp.h
>>>> index 3a47f52959a8..5b37f7bf2060 100644
>>>> --- a/arch/loongarch/include/asm/smp.h
>>>> +++ b/arch/loongarch/include/asm/smp.h
>>>> @@ -18,6 +18,7 @@ struct smp_ops {
>>>> void (*init_ipi)(void);
>>>> void (*send_ipi_single)(int cpu, unsigned int action);
>>>> void (*send_ipi_mask)(const struct cpumask *mask, unsigned int action);
>>>> + bool (*vcpu_is_preempted)(int cpu);
>>>> };
>>>> extern struct smp_ops mp_ops;
>>>>
>>>> diff --git a/arch/loongarch/include/asm/spinlock.h b/arch/loongarch/include/asm/spinlock.h
>>>> index 7cb3476999be..c001cef893aa 100644
>>>> --- a/arch/loongarch/include/asm/spinlock.h
>>>> +++ b/arch/loongarch/include/asm/spinlock.h
>>>> @@ -5,6 +5,11 @@
>>>> #ifndef _ASM_SPINLOCK_H
>>>> #define _ASM_SPINLOCK_H
>>>>
>>>> +#ifdef CONFIG_PARAVIRT
>>>> +#define vcpu_is_preempted vcpu_is_preempted
>>>> +bool vcpu_is_preempted(int cpu);
>>>> +#endif
>>> Maybe paravirt.h is a better place?
>>
>> It is actually a little strange to add macro CONFIG_PARAVIRT in file
>> asm/spinlock.h
>>
>> vcpu_is_preempted is originally defined in header file
>> include/linux/sched.h like this
>> #ifndef vcpu_is_preempted
>> static inline bool vcpu_is_preempted(int cpu)
>> {
>> return false;
>> }
>> #endif
>>
>> that requires that header file is included before sched.h, file
>> asm/spinlock.h can meet this requirement, however header file paravirt.h
>> maybe it is not included before sched.h in generic.
>>
>> Here vcpu_is_preempted definition is added before the following including.
>> #include <asm/processor.h>
>> #include <asm/qspinlock.h>
>> #include <asm/qrwlock.h>
>> Maybe it is better to be added after the above header files including
>> sentences, but need further investigation.
> powerpc put it in paravirt.h, so I think it is possible.
paravirt.h is included by header file asm/qspinlock.h on powerpc,
however it is not so on loongarch :)
# grep paravirt.h arch/powerpc/* -r
arch/powerpc/include/asm/paravirt_api_clock.h:#include <asm/paravirt.h>
arch/powerpc/include/asm/qspinlock.h:#include <asm/paravirt.h>
arch/powerpc/include/asm/simple_spinlock.h:#include <asm/paravirt.h>
$ grep paravirt.h arch/loongarch/* -r
arch/loongarch/include/asm/paravirt_api_clock.h:#include <asm/paravirt.h>
>
>>>
>>>> +
>>>> #include <asm/processor.h>
>>>> #include <asm/qspinlock.h>
>>>> #include <asm/qrwlock.h>
>>>> diff --git a/arch/loongarch/kernel/paravirt.c b/arch/loongarch/kernel/paravirt.c
>>>> index b1b51f920b23..b99404b6b13f 100644
>>>> --- a/arch/loongarch/kernel/paravirt.c
>>>> +++ b/arch/loongarch/kernel/paravirt.c
>>>> @@ -52,6 +52,13 @@ static u64 paravt_steal_clock(int cpu)
>>>> #ifdef CONFIG_SMP
>>>> static struct smp_ops native_ops;
>>>>
>>>> +static bool pv_vcpu_is_preempted(int cpu)
>>>> +{
>>>> + struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
>>>> +
>>>> + return !!(src->preempted & KVM_VCPU_PREEMPTED);
>>>> +}
>>>> +
>>>> static void pv_send_ipi_single(int cpu, unsigned int action)
>>>> {
>>>> int min, old;
>>>> @@ -308,6 +315,9 @@ 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_HINT))
>>>> + mp_ops.vcpu_is_preempted = pv_vcpu_is_preempted;
>>>> #endif
>>>>
>>>> static_call_update(pv_steal_clock, paravt_steal_clock);
>>>> @@ -332,3 +342,9 @@ int __init pv_spinlock_init(void)
>>>>
>>>> return 0;
>>>> }
>>>> +
>>>> +bool notrace vcpu_is_preempted(int cpu)
>>>> +{
>>>> + return mp_ops.vcpu_is_preempted(cpu);
>>>> +}
>>>
>>> We can simplify the whole patch like this, then we don't need to touch
>>> smp.c, and we can merge Patch-2/3.
>>>
>>> +bool notrace vcpu_is_preempted(int cpu)
>>> +{
>>> + if (!kvm_para_has_feature(KVM_FEATURE_PREEMPT_HINT))
>>> + return false;
>>> + else {
>>> + struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
>>> + return !!(src->preempted & KVM_VCPU_PREEMPTED);
>>> + }
>>> +}
>> 1. there is assembly output about relative vcpu_is_preempted
>> <loongson_vcpu_is_preempted>:
>> move $r4,$r0
>> jirl $r0,$r1,0
>>
>> <pv_vcpu_is_preempted>:
>> pcalau12i $r13,8759(0x2237)
>> slli.d $r4,$r4,0x3
>> addi.d $r13,$r13,-1000(0xc18)
>> ldx.d $r13,$r13,$r4
>> pcalau12i $r12,5462(0x1556)
>> addi.d $r12,$r12,384(0x180)
>> add.d $r12,$r13,$r12
>> ld.bu $r4,$r12,16(0x10)
>> andi $r4,$r4,0x1
>> jirl $r0,$r1,0
>>
>> <vcpu_is_preempted>:
>> pcalau12i $r12,8775(0x2247)
>> ld.d $r12,$r12,-472(0xe28)
>> jirl $r0,$r12,0
>> andi $r0,$r0,0x0
>>
>> <vcpu_is_preempted_new>:
>> pcalau12i $r12,8151(0x1fd7)
>> ld.d $r12,$r12,-1008(0xc10)
>> bstrpick.d $r12,$r12,0x1a,0x1a
>> beqz $r12,188(0xbc) # 900000000024ec60
>> pcalau12i $r12,11802(0x2e1a)
>> addi.d $r12,$r12,-1400(0xa88)
>> ldptr.w $r14,$r12,36(0x24)
>> beqz $r14,108(0x6c) # 900000000024ec20
>> addi.w $r13,$r0,1(0x1)
>> bne $r14,$r13,164(0xa4) # 900000000024ec60
>> ldptr.w $r13,$r12,40(0x28)
>> bnez $r13,24(0x18) # 900000000024ebdc
>> lu12i.w $r14,262144(0x40000)
>> ori $r14,$r14,0x4
>> cpucfg $r14,$r14
>> slli.w $r13,$r14,0x0
>> st.w $r14,$r12,40(0x28)
>> bstrpick.d $r13,$r13,0x3,0x3
>> beqz $r13,128(0x80) # 900000000024ec60
>> pcalau12i $r13,8759(0x2237)
>> slli.d $r4,$r4,0x3
>> addi.d $r13,$r13,-1000(0xc18)
>> ldx.d $r13,$r13,$r4
>> pcalau12i $r12,5462(0x1556)
>> addi.d $r12,$r12,384(0x180)
>> add.d $r12,$r13,$r12
>> ld.bu $r4,$r12,16(0x10)
>> andi $r4,$r4,0x1
>> jirl $r0,$r1,0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> lu12i.w $r13,262144(0x40000)
>> cpucfg $r13,$r13
>> lu12i.w $r15,1237(0x4d5)
>> ori $r15,$r15,0x64b
>> slli.w $r13,$r13,0x0
>> bne $r13,$r15,-124(0x3ff84) # 900000000024ebb8
>> addi.w $r13,$r0,1(0x1)
>> st.w $r13,$r12,36(0x24)
>> b -128(0xfffff80) # 900000000024ebc0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> andi $r0,$r0,0x0
>> move $r4,$r0
>> jirl $r0,$r1,0
>>
>> With vcpu_is_preempted(), there is one memory load and one jirl jump,
>> with vcpu_is_preempted_new(), there is two memory load and two beq
>> compare instructions.
> Is vcpu_is_preempted() performance critical (we need performance data
> here)? It seems the powerpc version is also complex.
>
>>
>> 2. In some scenery such nr_cpus == 1, loongson_vcpu_is_preempted() is
>> better than pv_vcpu_is_preempted() even if the preempt feature is enabled.
> In your original patch, "mp_ops.vcpu_is_preempted =
> pv_vcpu_is_preempted" if the preempt feature is enabled. Why is
> loongson_vcpu_is_preempted() called when nr_cpus=1?
>
> Huacai
>
>>
>> Regards
>> Bibo Mao
>>> Huacai
>>>
>>>> +EXPORT_SYMBOL(vcpu_is_preempted);
>>>> diff --git a/arch/loongarch/kernel/smp.c b/arch/loongarch/kernel/smp.c
>>>> index 46036d98da75..f04192fedf8d 100644
>>>> --- a/arch/loongarch/kernel/smp.c
>>>> +++ b/arch/loongarch/kernel/smp.c
>>>> @@ -307,10 +307,16 @@ static void loongson_init_ipi(void)
>>>> panic("IPI IRQ request failed\n");
>>>> }
>>>>
>>>> +static bool loongson_vcpu_is_preempted(int cpu)
>>>> +{
>>>> + return false;
>>>> +}
>>>> +
>>>> struct smp_ops mp_ops = {
>>>> .init_ipi = loongson_init_ipi,
>>>> .send_ipi_single = loongson_send_ipi_single,
>>>> .send_ipi_mask = loongson_send_ipi_mask,
>>>> + .vcpu_is_preempted = loongson_vcpu_is_preempted,
>>>> };
>>>>
>>>> static void __init fdt_smp_setup(void)
>>>> --
>>>> 2.39.3
>>>>
>>>>
>>
>>
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