linux-kernel - Re: [PATCH] KVM: x86: Use div64_ul instead of do

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Date:   Fri, 17 Dec 2021 09:44:55 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     cgel.zte@...il.com
Cc:     seanjc@...gle.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Changcheng Deng <deng.changcheng@....com.cn>,
        Zeal Robot <zealci@....com.cn>
Subject: Re: [PATCH] KVM: x86: Use div64_ul instead of do_div

On 12/17/21 09:41, cgel.zte@...il.com wrote:
> From: Changcheng Deng <deng.changcheng@....com.cn>
> 
> do_div() does a 64-by-32 division. Here the divisor is an unsigned long
> which on some platforms is 64 bit wide. So use div64_ul instead of do_div
> to avoid a possible truncation.
> 
> Reported-by: Zeal Robot <zealci@....com.cn>
> Signed-off-by: Changcheng Deng <deng.changcheng@....com.cn>
> ---
>   arch/x86/kvm/lapic.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index c5028e6b0f96..3b629870632c 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1707,7 +1707,7 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
>   	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
>   
>   	ns = (tscdeadline - guest_tsc) * 1000000ULL;
> -	do_div(ns, this_tsc_khz);
> +	ns = div64_ul(ns, this_tsc_khz);
>   
>   	if (likely(tscdeadline > guest_tsc) &&
>   	    likely(ns > apic->lapic_timer.timer_advance_ns)) {
> 

You could change this_tsc_khz to u32 instead, it's assigned from a 
32-bit value.

Using div64_ul would be unnecessary and less efficient.

Paolo