xen/arm: avoid overflow when setting vtimer in context switch

virt_vtimer_save() will calculate the next deadline when the vCPU is
scheduled out. At the moment, Xen will use the following equation:

  virt_timer.cval + virt_time_base.offset - boot_count

The three values are 64-bit and one (cval) is controlled by domain. In
theory, it would be possible that the domain has started a long time
after the system boot. So virt_time_base.offset - boot_count may be a
large numbers.

This means a domain may inadvertently set a cval so the result would
overflow. Consequently, the deadline would be set very far in the
future. This could result to loss of timer interrupts or the vCPU
getting block "forever".

One way to solve the problem, would be to separately
   1) compute when the domain was created in ns
   2) convert cval to ns
   3) Add 1 and 2 together

The first part of the equation never change (the value is set/known at
domain creation). So take the opportunity to store it in domain structure.

Signed-off-by: Jiamei Xie <jiamei.xie@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

diff --git a/xen/arch/arm/include/asm/domain.h b/xen/arch/arm/include/asm/domain.h
index ed63c2b6f91f6171db3a5f4bc9e66f4175cfdef5..cd9ce19b4b4169647dd052bb457bc000e82307ad 100644 (file)
--- 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 6b78fea77d7098d1817a2870bc3d3e69426be65e..aeaea78e4c783412fc2d187114f074fe3f33bedb 100644 (file)
--- 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));
     }
 }