Re: [PATCH] KVM: x86: Use div64_ul instead of do_div

[Paolo Bonzini]

On 12/17/21 09:41, cgel.zte@xxxxxxxxx wrote:
> From: Changcheng Deng <deng.changcheng@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Signed-off-by: Changcheng Deng <deng.changcheng@xxxxxxxxxx>
> ---
>   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