Re: [PATCH v4 18/47] x86/kvm: Get local APIC bus frequency from PV CPUID Timing Info

[Sean Christopherson <seanjc@google.com>]

On Fri, May 29, 2026, sashiko-bot@kernel.org wrote:
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 4fe9c69bf40b3..c1139182121d5 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> 
> [ ... ]
> 
> > @@ -1039,6 +1040,13 @@ static void __init kvm_init_platform(void)
> >  			x86_init.hyper.get_tsc_khz = kvm_get_tsc_khz;
> >  			x86_init.hyper.get_cpu_khz = kvm_get_tsc_khz;
> >  		}
> > +
> > +#ifdef CONFIG_X86_LOCAL_APIC
> > +		/* The leaf also includes the local APIC bus/timer frequency.*/
> > +		apic_khz = cpuid_ebx(timing_info_leaf);
> > +		if (apic_khz)
> > +	               lapic_timer_period = apic_khz * 1000 / HZ;
> 
> [Severity: High]
> Could the intermediate multiplication of apic_khz * 1000 overflow here?
> 
> Since apic_khz is a u32, the multiplication is evaluated in 32-bit unsigned
> arithmetic. If the hypervisor reports an APIC timer frequency greater than
> ~4.29 GHz (apic_khz > 4294967), this calculation will silently overflow.

Hmm, easy enough to use mul_u64_u32_div() (I think that's the write helper for
this?).

But this problem pre-exits in almost every other path that sets lapic_timer_period.
So while I tried to avoid doing yet more tangentially related cleanup, it seems
like adding a helper to set lapic_timer_period is the way to go.  That would also
allow making lapic_timer_period local to arch/x86/kernel/apic/apic.c.

*sigh*