[Xen-devel] [PATCH 1/2] x86/hvm: prevent guest's timers from going backwards, when timer_mode=0

[Kouya Shimura <kouya@xxxxxxxxxxxxxx>]

In the case of delay_for_missed_ticks mode (timer_mode=0), the guest
virtual time goes backwards when the vcpu is rescheduled. Therefore
guest's HW timer might go backwards, too.

Case 1. SMP guest:
 1) vcpu#1 is de-scheduled and the guest time freezes. (TIME 0:0.010)
 2) vcpu#2 access a timer (0:0.020)
 3) vcpu#1 is re-scheduled and the guest time thaws.   (0:0.030->0:0.010)
 4) vcpu#2 access a timer (0:0.015) // Backwards!!!

Case 2. asynchronous callback:
 1) vcpu#1 is de-scheduled and the guest time freezes. (0:0.010)
 2) pmt_timer_callback() is invoked (0:0.025)
 3) vcpu#1 is re-scheduled and the guest time thaws.   (0:0.030->0:0.010)
 4) vcpu#1 access the PM-TIMER (0:0.015) // Backwards!!!

This patch affects only delay_for_missed_ticks mode (timer_mode=0) and
ensures the monotonicity of the following timers:
 - PIT
 - HPET
 - ACPI PM-TIMER

The following timers are OK since a vcpu never access the other vcpu's timer.
 - Local APIC ( has some callbacks but it's called from pt_intr_post )
 - TSC
Just in case, these timers can use the new function hvm_get_base_time() too,
but doesn't. It's a little bit less efficient than hvm_get_guest_time().

Also, tidy up virtual platform timer code.

Signed-off-by: Kouya Shimura <kouya@xxxxxxxxxxxxxx>
---
 xen/arch/x86/hvm/hpet.c       |    2 +-
 xen/arch/x86/hvm/i8254.c      |    2 +-
 xen/arch/x86/hvm/pmtimer.c    |    2 +-
 xen/arch/x86/hvm/vpt.c        |   93 ++++++++++++++++++++++++++++++++++-------
 xen/include/asm-x86/hvm/hvm.h |    2 +-
 5 files changed, 82 insertions(+), 19 deletions(-)

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index 4b4b905..fa44d37 100644
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -39,7 +39,7 @@
 /* Frequency_of_Xen_systeme_time / frequency_of_HPET = 16 */
 #define STIME_PER_HPET_TICK 16
 #define guest_time_hpet(hpet) \
-    (hvm_get_guest_time(vhpet_vcpu(hpet)) / STIME_PER_HPET_TICK)
+    (hvm_get_base_time(vhpet_vcpu(hpet)) / STIME_PER_HPET_TICK)
 
 #define HPET_TN_INT_ROUTE_CAP_SHIFT 32
 #define HPET_TN_CFG_BITS_READONLY_OR_RESERVED (HPET_TN_RESERVED | \
diff --git a/xen/arch/x86/hvm/i8254.c b/xen/arch/x86/hvm/i8254.c
index c0d6bc2..c45ed88 100644
--- a/xen/arch/x86/hvm/i8254.c
+++ b/xen/arch/x86/hvm/i8254.c
@@ -54,7 +54,7 @@ static int handle_speaker_io(
     int dir, uint32_t port, uint32_t bytes, uint32_t *val);
 
 #define get_guest_time(v) \
-   (is_hvm_vcpu(v) ? hvm_get_guest_time(v) : (u64)get_s_time())
+   (is_hvm_vcpu(v) ? hvm_get_base_time(v) : (u64)get_s_time())
 
 static int pit_get_count(PITState *pit, int channel)
 {
diff --git a/xen/arch/x86/hvm/pmtimer.c b/xen/arch/x86/hvm/pmtimer.c
index 01ae31d..5c25cfb 100644
--- a/xen/arch/x86/hvm/pmtimer.c
+++ b/xen/arch/x86/hvm/pmtimer.c
@@ -93,7 +93,7 @@ static void pmt_update_time(PMTState *s)
     ASSERT(spin_is_locked(&s->lock));
 
     /* Update the timer */
-    curr_gtime = hvm_get_guest_time(s->vcpu);
+    curr_gtime = hvm_get_base_time(s->vcpu);
     tmp = ((curr_gtime - s->last_gtime) * s->scale) + s->not_accounted;
     s->not_accounted = (uint32_t)tmp;
     tmr_val += tmp >> 32;
diff --git a/xen/arch/x86/hvm/vpt.c b/xen/arch/x86/hvm/vpt.c
index 46d3ec6..7a3edf3 100644
--- a/xen/arch/x86/hvm/vpt.c
+++ b/xen/arch/x86/hvm/vpt.c
@@ -36,6 +36,19 @@ void hvm_init_guest_time(struct domain *d)
     pl->last_guest_time = 0;
 }
 
+static inline u64 pt_now(struct pl_time *pl, struct vcpu *v)
+{
+    u64 now = get_s_time() + pl->stime_offset;
+
+    ASSERT(spin_is_locked(&pl->pl_time_lock));
+
+    if ( (int64_t)(now - pl->last_guest_time) > 0 )
+        pl->last_guest_time = now;
+    else
+        now = ++pl->last_guest_time;
+    return now + v->arch.hvm_vcpu.stime_offset;
+}
+
 u64 hvm_get_guest_time(struct vcpu *v)
 {
     struct pl_time *pl = &v->domain->arch.hvm_domain.pl_time;
@@ -45,19 +58,33 @@ u64 hvm_get_guest_time(struct vcpu *v)
     ASSERT(is_hvm_vcpu(v));
 
     spin_lock(&pl->pl_time_lock);
-    now = get_s_time() + pl->stime_offset;
-    if ( (int64_t)(now - pl->last_guest_time) > 0 )
-        pl->last_guest_time = now;
-    else
-        now = ++pl->last_guest_time;
+    now = pt_now(pl, v);
     spin_unlock(&pl->pl_time_lock);
-
-    return now + v->arch.hvm_vcpu.stime_offset;
+    return now;
 }
 
-void hvm_set_guest_time(struct vcpu *v, u64 guest_time)
+/*
+ * This function is used to emulate HW timer counters. In the case of
+ * delay_for_missed_ticks mode, the guest time once goes backwards to
+ * the frozen time when the vcpu is rescheduled. To avoid decrement
+ * of a timer counter, return the frozen time while the vcpu is not
+ * being scheduled.
+ */
+u64 hvm_get_base_time(struct vcpu *v)
 {
-    v->arch.hvm_vcpu.stime_offset += guest_time - hvm_get_guest_time(v);
+    struct pl_time *pl = &v->domain->arch.hvm_domain.pl_time;
+    u64 now;
+
+    /* Called from device models shared with PV guests. Be careful. */
+    ASSERT(is_hvm_vcpu(v));
+
+    spin_lock(&pl->pl_time_lock);
+    if ( v->arch.hvm_vcpu.guest_time ) /* the guest time is frozen */
+        now = v->arch.hvm_vcpu.guest_time;
+    else
+        now = pt_now(pl, v);
+    spin_unlock(&pl->pl_time_lock);
+    return now;
 }
 
 static int pt_irq_vector(struct periodic_time *pt, enum hvm_intsrc src)
@@ -138,24 +165,62 @@ static void pt_process_missed_ticks(struct periodic_time 
*pt)
     pt->scheduled += missed_ticks * pt->period;
 }
 
+/*
+ * N.B. The following three functions, pt_freeze_time(),
+ * pt_thaw_time() and pt_step_time() never race with each others,
+ * but race with either hvm_get_guest_time() or hvm_get_base_time().
+ */
+
 static void pt_freeze_time(struct vcpu *v)
 {
+    struct pl_time *pl;
+
     if ( !mode_is(v->domain, delay_for_missed_ticks) )
         return;
 
-    v->arch.hvm_vcpu.guest_time = hvm_get_guest_time(v);
+    pl = &v->domain->arch.hvm_domain.pl_time;
+    spin_lock(&pl->pl_time_lock);
+    v->arch.hvm_vcpu.guest_time = pt_now(pl, v);
+    spin_unlock(&pl->pl_time_lock);
 }
 
 static void pt_thaw_time(struct vcpu *v)
 {
+    struct pl_time *pl;
+    u64 now, frozen_time = v->arch.hvm_vcpu.guest_time;
+
+#if 0 /* redundant */
     if ( !mode_is(v->domain, delay_for_missed_ticks) )
         return;
+#endif
 
-    if ( v->arch.hvm_vcpu.guest_time == 0 )
+    if ( frozen_time == 0 )
         return;
 
-    hvm_set_guest_time(v, v->arch.hvm_vcpu.guest_time);
+    ASSERT(mode_is(v->domain, delay_for_missed_ticks));
+
+    pl = &v->domain->arch.hvm_domain.pl_time;
+    spin_lock(&pl->pl_time_lock);
+    now = pt_now(pl, v);
+    v->arch.hvm_vcpu.stime_offset += frozen_time - now;
     v->arch.hvm_vcpu.guest_time = 0;
+    spin_unlock(&pl->pl_time_lock);
+}
+
+static void pt_step_time(struct vcpu *v, u64 guest_time)
+{
+    struct pl_time *pl;
+    u64 now;
+
+    if ( !mode_is(v->domain, delay_for_missed_ticks) )
+        return;
+
+    pl = &v->domain->arch.hvm_domain.pl_time;
+    spin_lock(&pl->pl_time_lock);
+    now = pt_now(pl, v);
+    if ( now < guest_time )
+        v->arch.hvm_vcpu.stime_offset += guest_time - now;
+    spin_unlock(&pl->pl_time_lock);
 }
 
 void pt_save_timer(struct vcpu *v)
@@ -341,9 +406,7 @@ void pt_intr_post(struct vcpu *v, struct hvm_intack intack)
         }
     }
 
-    if ( mode_is(v->domain, delay_for_missed_ticks) &&
-         (hvm_get_guest_time(v) < pt->last_plt_gtime) )
-        hvm_set_guest_time(v, pt->last_plt_gtime);
+    pt_step_time(v, pt->last_plt_gtime);
 
     cb = pt->cb;
     cb_priv = pt->priv;
diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
index 2fa2ea5..f4cd200 100644
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -226,8 +226,8 @@ void hvm_set_guest_tsc(struct vcpu *v, u64 guest_tsc);
 u64 hvm_get_guest_tsc(struct vcpu *v);
 
 void hvm_init_guest_time(struct domain *d);
-void hvm_set_guest_time(struct vcpu *v, u64 guest_time);
 u64 hvm_get_guest_time(struct vcpu *v);
+u64 hvm_get_base_time(struct vcpu *v);
 
 int vmsi_deliver(
     struct domain *d, int vector,
--
1.7.9.5



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