linux-kernel - Re: [PATCH 13/34] KVM: x86: hyper-v: optimize and cleanup kvm_hv_process

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Date:   Mon, 20 Nov 2023 15:26:08 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>, Jan Kara <jack@...e.cz>,
        Mirsad Todorovac <mirsad.todorovac@....unizg.hr>,
        Matthew Wilcox <willy@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Alexey Klimov <klimov.linux@...il.com>
Subject: Re: [PATCH 13/34] KVM: x86: hyper-v: optimize and cleanup
 kvm_hv_process_stimers()

Yury Norov <yury.norov@...il.com> writes:

> The function traverses stimer_pending_bitmap n a for-loop bit by bit.
> We can do it faster by using atomic find_and_set_bit().
>
> While here, refactor the logic by decreasing indentation level
> and dropping 2nd check for stimer->config.enable.
>
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>  arch/x86/kvm/hyperv.c | 39 +++++++++++++++++++--------------------
>  1 file changed, 19 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 238afd7335e4..460e300b558b 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -870,27 +870,26 @@ void kvm_hv_process_stimers(struct kvm_vcpu *vcpu)
>  	if (!hv_vcpu)
>  		return;
>  
> -	for (i = 0; i < ARRAY_SIZE(hv_vcpu->stimer); i++)
> -		if (test_and_clear_bit(i, hv_vcpu->stimer_pending_bitmap)) {
> -			stimer = &hv_vcpu->stimer[i];
> -			if (stimer->config.enable) {
> -				exp_time = stimer->exp_time;
> -
> -				if (exp_time) {
> -					time_now =
> -						get_time_ref_counter(vcpu->kvm);
> -					if (time_now >= exp_time)
> -						stimer_expiration(stimer);
> -				}
> -
> -				if ((stimer->config.enable) &&
> -				    stimer->count) {
> -					if (!stimer->msg_pending)
> -						stimer_start(stimer);
> -				} else
> -					stimer_cleanup(stimer);
> -			}
> +	for_each_test_and_clear_bit(i, hv_vcpu->stimer_pending_bitmap,
> +					ARRAY_SIZE(hv_vcpu->stimer)) {
> +		stimer = &hv_vcpu->stimer[i];
> +		if (!stimer->config.enable)
> +			continue;
> +
> +		exp_time = stimer->exp_time;
> +
> +		if (exp_time) {
> +			time_now = get_time_ref_counter(vcpu->kvm);
> +			if (time_now >= exp_time)
> +				stimer_expiration(stimer);
>  		}
> +
> +		if (stimer->count) {

You can't drop 'stimer->config.enable' check here as stimer_expiration()
call above actually changes it. This is done on purpose: oneshot timers
fire only once so 'config.enable' is reset to 0.

> +			if (!stimer->msg_pending)
> +				stimer_start(stimer);
> +		} else
> +			stimer_cleanup(stimer);
> +	}
>  }
>  
>  void kvm_hv_vcpu_uninit(struct kvm_vcpu *vcpu)

-- 
Vitaly