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Message-ID: <db8a26ca-8430-9e7d-4ad1-9b7743c4cfd1@loongson.cn>
Date: Wed, 19 Nov 2025 09:59:19 +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 2025/11/18 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.
>
>> +
>> #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.
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.
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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