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



Hi Jiami

Title: s/caclulations/calculations/

However, I think the title should mention the overflow rather than the extra calculations. The former is more important the latter.

On 27/06/2022 03:58, Jiamei Xie wrote:
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

This read strange to me. We want to remove the overflow completely not reducing it. The overflow is completely removed by converting the "offset - bount_count" to ns upfront.

AFAICT, the commit message doesn't explain that.

, 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

This link doesn't work. But I would personally remove it from the commit message (or add ---) because it doesn't bring value (this patch looks like a v2 to me).

---
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;

This should be s_time_t to match the argument of set_timer() and return of ticks_to_ns().

+    } vtimer_offset;

Why are you adding a new structure rather than re-using virt_timer_base?

+
      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;

Hmmm... I find odd to assign a field "nanoseconds" to "seconds". I would suggest to re-order so you first set nanoseconds and then set seconds.

This will make more obvious that this is not a mistake and "seconds" will be closer to the do_div() below.

      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));
      }
  }

Cheers,

--
Julien Grall



 


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