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>
(cherry picked from commit 6655eb81092a94e065fdcd0b47a1b1d69dc4e54c)

diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
index 0196951af4eb5f5ff40d56172bf652612ec2f3f1..23198d03969ceabac08941c70ad1d6ab6ba4fa5f 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));
     }
 }
 

diff --git a/xen/include/asm-arm/domain.h b/xen/include/asm-arm/domain.h
index 9b3647587a045c18a925acab2f963d2606d056a3..7f8ddd3f5c3b589f9bbf25b78bd42bfa7730d17a 100644 (file)
--- a/xen/include/asm-arm/domain.h
+++ b/xen/include/asm-arm/domain.h
@@ -58,6 +58,7 @@ struct arch_domain
 
     struct {
         uint64_t offset;
+        s_time_t nanoseconds;
     } virt_timer_base;
 
     struct vgic_dist vgic;