[PATCH v3] xen/arm: avoid overflow when setting vtimer in context switch

[Jiamei Xie]

virt_vtimer_save is calculating the new time for the vtimer in:
"v->arch.virt_timer.cval + v->domain->arch.virt_timer_base.offset
- boot_count".
In this formula, "cval + offset" might cause uint64_t overflow.
Changing it to "ticks_to_ns(v->domain->arch.virt_timer_base.offset -
boot_count) + ticks_to_ns(v->arch.virt_timer.cval)" can avoid overflow,
and "ticks_to_ns(arch.virt_timer_base.offset - boot_count)" will be
always the same, which has been caculated in domain_vtimer_init.
Introduce a new field virt_timer_base.nanoseconds to store this value
for arm in struct arch_domain, so we can use it directly.

Signed-off-by: Jiamei Xie <jiamei....@arm.com>
Change-Id: Ib80cee51eaf844661e6f92154a0339ad2a652f9b
---
was "xen/arm: avoid vtimer flip-flop transition in context switch".
v3 changes:
-re-write commit message
-store nanoseconds in virt_timer_base instead of adding a new structure
-assign to nanoseconds first, then seconds
---
 xen/arch/arm/include/asm/domain.h | 1 +
 xen/arch/arm/vtimer.c             | 9 ++++++---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/include/asm/domain.h 
b/xen/arch/arm/include/asm/domain.h
index ed63c2b6f9..cd9ce19b4b 100644
--- a/xen/arch/arm/include/asm/domain.h
+++ b/xen/arch/arm/include/asm/domain.h
@@ -71,6 +71,7 @@ struct arch_domain
 
     struct {
         uint64_t offset;
+        s_time_t nanoseconds;
     } virt_timer_base;
 
     struct vgic_dist vgic;
diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
index 6b78fea77d..aeaea78e4c 100644
--- a/xen/arch/arm/vtimer.c
+++ b/xen/arch/arm/vtimer.c
@@ -63,7 +63,9 @@ static void virt_timer_expired(void *data)
 int domain_vtimer_init(struct domain *d, struct xen_arch_domainconfig *config)
 {
     d->arch.virt_timer_base.offset = get_cycles();
-    d->time_offset.seconds = ticks_to_ns(d->arch.virt_timer_base.offset - 
boot_count);
+    d->arch.virt_timer_base.nanoseconds =
+        ticks_to_ns(d->arch.virt_timer_base.offset - boot_count);
+    d->time_offset.seconds = d->arch.virt_timer_base.nanoseconds;
     do_div(d->time_offset.seconds, 1000000000);
 
     config->clock_frequency = timer_dt_clock_frequency;
@@ -144,8 +146,9 @@ void virt_timer_save(struct vcpu *v)
     if ( (v->arch.virt_timer.ctl & CNTx_CTL_ENABLE) &&
          !(v->arch.virt_timer.ctl & CNTx_CTL_MASK))
     {
-        set_timer(&v->arch.virt_timer.timer, 
ticks_to_ns(v->arch.virt_timer.cval +
-                  v->domain->arch.virt_timer_base.offset - boot_count));
+        set_timer(&v->arch.virt_timer.timer,
+                  v->domain->arch.virt_timer_base.nanoseconds +
+                  ticks_to_ns(v->arch.virt_timer.cval));
     }
 }
 
-- 
2.25.1