linux-kernel - [PATCH 5.0 021/122] KVM: x86: Consider LAPIC TSC-Deadline timer expired if deadline too short

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Message-Id: <20190506143056.649682511@linuxfoundation.org>
Date:   Mon,  6 May 2019 16:31:19 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Joao Martins <joao.m.martins@...cle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.0 021/122] KVM: x86: Consider LAPIC TSC-Deadline timer expired if deadline too short

From: Liran Alon <liran.alon@...cle.com>

commit c09d65d9eab69985c75f98ed64541229f6fa9aa6 upstream.

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@...cle.com>
Signed-off-by: Liran Alon <liran.alon@...cle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/kvm/lapic.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1543,9 +1543,12 @@ static void start_sw_tscdeadline(struct

 	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,