linux-kernel - Re: [PATCH v4 1/3] LoongArch: KVM: Remove SW timer switch when vcpu is halt polling

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Date:   Mon, 4 Dec 2023 16:48:01 +0800
From:   zhaotianrui <zhaotianrui@...ngson.cn>
To:     Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>
Cc:     WANG Xuerui <kernel@...0n.name>, kvm@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] LoongArch: KVM: Remove SW timer switch when vcpu
 is halt polling

Reviewed-by: Tianrui Zhao <zhaotianrui@...ngson.cn>

在 2023/11/16 上午10:30, Bibo Mao 写道:
> With halt-polling supported, there is checking for pending events
> or interrupts when vcpu executes idle instruction. Pending interrupts
> include injected SW interrupts and passthrough HW interrupts, such as
> HW timer interrupts, since HW timer works still even if vcpu exists
> from VM mode.
>
> Since HW timer pending interrupt can be set directly with CSR status
> register, and pending HW timer interrupt checking is used in vcpu block
> checking function, it is not necessary to switch to sw timer during
> halt-polling. This patch adds preemption disabling in function
> kvm_cpu_has_pending_timer, and removes SW timer switching in idle
> instruction emulation function.
>
> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
> ---
>   arch/loongarch/kvm/exit.c  | 13 ++-----------
>   arch/loongarch/kvm/timer.c | 13 ++++++++++---
>   arch/loongarch/kvm/vcpu.c  |  9 ++++++++-
>   3 files changed, 20 insertions(+), 15 deletions(-)
>
> diff --git a/arch/loongarch/kvm/exit.c b/arch/loongarch/kvm/exit.c
> index ce8de3fa472c..e708a1786d6b 100644
> --- a/arch/loongarch/kvm/exit.c
> +++ b/arch/loongarch/kvm/exit.c
> @@ -200,17 +200,8 @@ int kvm_emu_idle(struct kvm_vcpu *vcpu)
>   	++vcpu->stat.idle_exits;
>   	trace_kvm_exit_idle(vcpu, KVM_TRACE_EXIT_IDLE);
>   
> -	if (!kvm_arch_vcpu_runnable(vcpu)) {
> -		/*
> -		 * Switch to the software timer before halt-polling/blocking as
> -		 * the guest's timer may be a break event for the vCPU, and the
> -		 * hypervisor timer runs only when the CPU is in guest mode.
> -		 * Switch before halt-polling so that KVM recognizes an expired
> -		 * timer before blocking.
> -		 */
> -		kvm_save_timer(vcpu);
> -		kvm_vcpu_block(vcpu);
> -	}
> +	if (!kvm_arch_vcpu_runnable(vcpu))
> +		kvm_vcpu_halt(vcpu);
>   
>   	return EMULATE_DONE;
>   }
> diff --git a/arch/loongarch/kvm/timer.c b/arch/loongarch/kvm/timer.c
> index 284bf553fefe..437e960d8fdb 100644
> --- a/arch/loongarch/kvm/timer.c
> +++ b/arch/loongarch/kvm/timer.c
> @@ -155,11 +155,18 @@ static void _kvm_save_timer(struct kvm_vcpu *vcpu)
>   		 */
>   		hrtimer_cancel(&vcpu->arch.swtimer);
>   		hrtimer_start(&vcpu->arch.swtimer, expire, HRTIMER_MODE_ABS_PINNED);
> -	} else
> +	} else if (vcpu->stat.generic.blocking) {
>   		/*
> -		 * Inject timer interrupt so that hall polling can dectect and exit
> +		 * Inject timer interrupt so that hall polling can dectect and
> +		 * exit.
> +		 * VCPU is scheduled out already and sleeps in rcuwait queue and
> +		 * will not poll pending events again. kvm_queue_irq is not
> +		 * enough, hrtimer swtimer should be used here.
>   		 */
> -		kvm_queue_irq(vcpu, INT_TI);
> +		expire = ktime_add_ns(ktime_get(), 10);  // 10ns is enough here?
> +		vcpu->arch.expire = expire;
> +		hrtimer_start(&vcpu->arch.swtimer, expire, HRTIMER_MODE_ABS_PINNED);
> +	}
>   }
>   
>   /*
> diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
> index 73d0c2b9c1a5..42663a345bd1 100644
> --- a/arch/loongarch/kvm/vcpu.c
> +++ b/arch/loongarch/kvm/vcpu.c
> @@ -187,8 +187,15 @@ int kvm_arch_vcpu_ioctl_translate(struct kvm_vcpu *vcpu,
>   
>   int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu)
>   {
> -	return kvm_pending_timer(vcpu) ||
> +	int ret;
> +
> +	/* protect from TOD sync and vcpu_load/put */
> +	preempt_disable();
> +	ret = kvm_pending_timer(vcpu) ||
>   		kvm_read_hw_gcsr(LOONGARCH_CSR_ESTAT) & (1 << INT_TI);
> +	preempt_enable();
> +
> +	return ret;
>   }
>   
>   int kvm_arch_vcpu_dump_regs(struct kvm_vcpu *vcpu)