Re: [PATCH 1/2] KVM: LAPIC: Optimize timer latency consider world switch time

[Paolo Bonzini]

On 30/05/19 21:36, Sean Christopherson wrote:
>> +u32 __read_mostly vmentry_lapic_timer_advance_ns = 0;
>> +module_param(vmentry_lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
> Hmm, an interesting idea would be to have some way to "lock" this param,
> e.g. setting bit 0 locks the param.  That would allow KVM to calculate the
> cycles value to avoid the function call and the MUL+DIV.  If I'm not
> mistaken, vcpu->arch.virtual_tsc_khz is set only in kvm_set_tsc_khz().

I would just make it read-only.  But I'm afraid we're entering somewhat
dangerous territory.  There is a risk that the guest ends up entering
the interrupt handler before the TSC deadline has actually expired, and
there would be no way to know what would happen; even guest hangs are
possible.

Paolo