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KVM: x86: Consider LAPIC TSC-Deadline timer expired if deadline too s…
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…hort

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>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Liran Alon authored and Paolo Bonzini committed Apr 18, 2019
1 parent 78671ab commit c09d65d
Showing 1 changed file with 6 additions and 3 deletions.
9 changes: 6 additions & 3 deletions arch/x86/kvm/lapic.c
Original file line number Diff line number Diff line change
Expand Up @@ -1542,9 +1542,12 @@ 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);

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,
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