Re: [PATCH] KVM: x86: Consider LAPIC TSC-Deadline timer expired if deadline too short

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Apr 16, 2019 at 08:36:34PM +0300, Liran Alon wrote:
> If guest sets MSR_IA32_TSCDEADLINE to value such that in host
> time-domain it's shorter than lapic_timer_advance_ns, we can
> reach a case that we call hrtimer_start() with expiration time set at
> the past.
> 
> Because lapic_timer.timer is init with HRTIMER_MODE_ABS_PINNED, it
> is not allowed to run in softirq and therefore will never expire.
> 
> To avoid such a scenario, verify that deadline expiration time is set on
> host time-domain further than (now + lapic_timer_advance_ns).
> 
> A future patch can also consider adding a min_timer_deadline_ns module parameter,
> similar to min_timer_period_us to avoid races that amount of ns it takes
> to run logic could still call hrtimer_start() with expiration timer set
> at the past.
> 
> Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
>  arch/x86/kvm/lapic.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9f089e2e09d0..fbd37e07e90d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1538,9 +1538,12 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
>  
>  	now = ktime_get();
>  	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> -	if (likely(tscdeadline > guest_tsc)) {
> -		ns = (tscdeadline - guest_tsc) * 1000000ULL;
> -		do_div(ns, this_tsc_khz);
> +
> +	ns = (tscdeadline - guest_tsc) * 1000000ULL;
> +	do_div(ns, this_tsc_khz);

This will do the division even if 'tscdeadline <= guest_tsc'.  I don't
see any reason to use ktime_sub_ns() since neither 'ns' or
'lapic_timer_advance_ns' is a ktime_t, e.g.:

	if (likely(tscdeadline > guest_tsc)) {
		ns = (tscdeadline - guest_tsc) * 1000000ULL;
		do_div(ns, this_tsc_khz);
		if (likely(ns > lapic_timer_advance_ns))
			ns -= lapic_timer_advance_ns;
		else
			ns = 0;
	} else {
		ns = 0;
	}
	if (ns) {
		now = ktime_get();
		expire = ktime_add_ns(now, ns);
		hrtimer_start(&apic->lapic_timer.timer,
				expire, HRTIMER_MODE_ABS_PINNED);
	} else
		apic_timer_expired(apic);

> +
> +	if (likely(tscdeadline > guest_tsc) &&
> +	    likely(ns > lapic_timer_advance_ns)) {
>  		expire = ktime_add_ns(now, ns);
>  		expire = ktime_sub_ns(expire, lapic_timer_advance_ns);
>  		hrtimer_start(&apic->lapic_timer.timer,
> -- 
> 2.20.1
>