[PATCH] xen/arm: avoid extra caclulations when setting vtimer in context switch

[Jiamei Xie <jiamei.xie@xxxxxxx>]

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 "v->domain->arch.virt_timer_base.offset - boot_count +
v->arch.virt_timer.cval" can reduce the possibility of overflow, and
"arch.virt_timer_base.offset - boot_count" will be always the same,
which has been caculated in domain_vtimer_init. Introduce a new field
vtimer_offset.nanoseconds to store this value for arm in struct
arch_domain, so we can use it directly and extra caclulations can be
avoided.

This patch is enlightened from [1].

Signed-off-by: Jiamei Xie <jiamei.xie@xxxxxxx>

[1] https://www.mail-archive.com/xen-devel@xxxxxxxxxxxxxxxxxxxx/msg123139.htm
---
xen/arch/arm/include/asm/domain.h | 4 ++++
 xen/arch/arm/vtimer.c             | 6 ++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/include/asm/domain.h 
b/xen/arch/arm/include/asm/domain.h
index ed63c2b6f9..94fe5b6444 100644
--- a/xen/arch/arm/include/asm/domain.h
+++ b/xen/arch/arm/include/asm/domain.h
@@ -73,6 +73,10 @@ struct arch_domain
         uint64_t offset;
     } virt_timer_base;
 
+    struct {
+        int64_t nanoseconds;
+    } vtimer_offset;
+
     struct vgic_dist vgic;
 
     struct vuart {
diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
index 6b78fea77d..54161e5fea 100644
--- a/xen/arch/arm/vtimer.c
+++ b/xen/arch/arm/vtimer.c
@@ -64,6 +64,7 @@ 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.vtimer_offset.nanoseconds = d->time_offset.seconds;
     do_div(d->time_offset.seconds, 1000000000);
 
     config->clock_frequency = timer_dt_clock_frequency;
@@ -144,8 +145,9 @@ 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.vtimer_offset.nanoseconds +
+                  ticks_to_ns(v->arch.virt_timer.cval));
     }
 }
 
-- 
2.25.1