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Message-ID: <CAAhV-H7uU6U9okJKqkUEXRT9OATpdMRgzZ8a7x0Xr6_bzD-x4A@mail.gmail.com>
Date: Wed, 19 Nov 2025 10:58:15 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
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 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.
> >
> >> +
> >> #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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