linux-kernel - Re: [PATCH] KVM: x86: replace do_div with div64

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Date:   Mon, 30 Oct 2023 09:12:53 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     "José Pekkarinen" <jose.pekkarinen@...hound.fi>
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com,
        skhan@...uxfoundation.org, x86@...nel.org, hpa@...or.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH] KVM: x86: replace do_div with div64_ul

On Sun, Oct 29, 2023, José Pekkarinen wrote:
> Reported by coccinelle, there is a do_div call that does
> 64-by-32 divisions even in 64bit platforms, this patch will
> move it to div64_ul macro that will decide the correct
> division function for the platform underneath. The output
> the warning follows:
> 
> arch/x86/kvm/lapic.c:1948:1-7: WARNING: do_div() does a 64-by-32 division, please consider using div64_ul instead.
> 
> Signed-off-by: José Pekkarinen <jose.pekkarinen@...hound.fi>
> ---
>  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 3e977dbbf993..0b90c6ad5091 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1945,7 +1945,7 @@ 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);
> +	div64_ul(ns, this_tsc_khz);

Well this is silly, virtual_tsc_khz is a u32.

	unsigned long this_tsc_khz = vcpu->arch.virtual_tsc_khz;

struct kvm_vcpu_arch { 

	...

	u32 virtual_tsc_khz;

}

I assume this will make coccinelle happy?

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 245b20973cae..31e9c84b8791 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1932,7 +1932,7 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
        u64 ns = 0;
        ktime_t expire;
        struct kvm_vcpu *vcpu = apic->vcpu;
-       unsigned long this_tsc_khz = vcpu->arch.virtual_tsc_khz;
+       u32 this_tsc_khz = vcpu->arch.virtual_tsc_khz;
        unsigned long flags;
        ktime_t now;