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Re: [Xen-devel] [PATCH v3 1/2] xen/arm: remove physical timer offset



Hi Jeff,

On 11/12/2019 21:13, Jeff Kubascik wrote:
The physical timer traps apply an offset so that time starts at 0 for
the guest. However, this offset is not currently applied to the physical
counter. Per the ARMv8 Reference Manual (ARM DDI 0487E.a), section
D11.2.4 Timers, the "Offset" between the counter and timer should be
zero for a physical timer. This removes the offset to make the timer and
counter consistent.

This also cleans up the physical timer implementation to better match
the virtual timer - both cval's now hold the hardware value.

Signed-off-by: Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>
---
  xen/arch/arm/vtimer.c        | 34 ++++++++++++++++++----------------
  xen/include/asm-arm/domain.h |  3 ---
  2 files changed, 18 insertions(+), 19 deletions(-)

diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
index e6aebdac9e..21b98ec20a 100644
--- a/xen/arch/arm/vtimer.c
+++ b/xen/arch/arm/vtimer.c
@@ -62,7 +62,6 @@ static void virt_timer_expired(void *data)
int domain_vtimer_init(struct domain *d, struct xen_arch_domainconfig *config)
  {
-    d->arch.phys_timer_base.offset = NOW();
      d->arch.virt_timer_base.offset = READ_SYSREG64(CNTPCT_EL0);
      d->time_offset_seconds = ticks_to_ns(d->arch.virt_timer_base.offset - 
boot_count);
      do_div(d->time_offset_seconds, 1000000000);
@@ -108,7 +107,6 @@ int vcpu_vtimer_init(struct vcpu *v)
init_timer(&t->timer, phys_timer_expired, t, v->processor);
      t->ctl = 0;
-    t->cval = NOW();
      t->irq = d0
          ? timer_get_irq(TIMER_PHYS_NONSECURE_PPI)
          : GUEST_TIMER_PHYS_NS_PPI;
@@ -167,6 +165,7 @@ void virt_timer_restore(struct vcpu *v)
  static bool vtimer_cntp_ctl(struct cpu_user_regs *regs, uint32_t *r, bool 
read)
  {
      struct vcpu *v = current;
+    s_time_t expires;
if ( !ACCESS_ALLOWED(regs, EL0PTEN) )
          return false;
@@ -184,8 +183,9 @@ static bool vtimer_cntp_ctl(struct cpu_user_regs *regs, 
uint32_t *r, bool read)
if ( v->arch.phys_timer.ctl & CNTx_CTL_ENABLE )
          {
-            set_timer(&v->arch.phys_timer.timer,
-                      v->arch.phys_timer.cval + 
v->domain->arch.phys_timer_base.offset);
+            expires = v->arch.phys_timer.cval > boot_count
+                      ? ticks_to_ns(v->arch.phys_timer.cval - boot_count) : 0;
+            set_timer(&v->arch.phys_timer.timer, expires);
          }
          else
              stop_timer(&v->arch.phys_timer.timer);
@@ -197,26 +197,27 @@ static bool vtimer_cntp_tval(struct cpu_user_regs *regs, 
uint32_t *r,
                               bool read)
  {
      struct vcpu *v = current;
-    s_time_t now;
+    uint64_t cntpct;
+    s_time_t expires;
if ( !ACCESS_ALLOWED(regs, EL0PTEN) )
          return false;
- now = NOW() - v->domain->arch.phys_timer_base.offset;
+    cntpct = get_cycles();
if ( read )
      {
-        *r = (uint32_t)(ns_to_ticks(v->arch.phys_timer.cval - now) & 
0xffffffffull);
+        *r = (uint32_t)((v->arch.phys_timer.cval - cntpct) & 0xffffffffull);
      }
      else
      {
-        v->arch.phys_timer.cval = now + ticks_to_ns(*r);
+        v->arch.phys_timer.cval = cntpct + *r;
          if ( v->arch.phys_timer.ctl & CNTx_CTL_ENABLE )
          {
              v->arch.phys_timer.ctl &= ~CNTx_CTL_PENDING;
-            set_timer(&v->arch.phys_timer.timer,
-                      v->arch.phys_timer.cval +
-                      v->domain->arch.phys_timer_base.offset);
+            expires = v->arch.phys_timer.cval > boot_count
+                      ? ticks_to_ns(v->arch.phys_timer.cval - boot_count) : 0;

You probably want a comment to explain why you set to 0 here.

+            set_timer(&v->arch.phys_timer.timer, expires);
          }
      }
      return true;
@@ -226,23 +227,24 @@ static bool vtimer_cntp_cval(struct cpu_user_regs *regs, 
uint64_t *r,
                               bool read)
  {
      struct vcpu *v = current;
+    s_time_t expires;
if ( !ACCESS_ALLOWED(regs, EL0PTEN) )
          return false;
if ( read )
      {
-        *r = ns_to_ticks(v->arch.phys_timer.cval);
+        *r = v->arch.phys_timer.cval;
      }
      else
      {
-        v->arch.phys_timer.cval = ticks_to_ns(*r);
+        v->arch.phys_timer.cval = *r;
          if ( v->arch.phys_timer.ctl & CNTx_CTL_ENABLE )
          {
              v->arch.phys_timer.ctl &= ~CNTx_CTL_PENDING;
-            set_timer(&v->arch.phys_timer.timer,
-                      v->arch.phys_timer.cval +
-                      v->domain->arch.phys_timer_base.offset);
+            expires = v->arch.phys_timer.cval > boot_count
+                      ? ticks_to_ns(v->arch.phys_timer.cval - boot_count) : 0;

Same here. But I am wondering whether we could factor this code in a function. This would avoid code duplication and make the code simpler.

This can be done as a follow-up as we may want to backport the fix.

+            set_timer(&v->arch.phys_timer.timer, expires);
          }
      }
      return true;
diff --git a/xen/include/asm-arm/domain.h b/xen/include/asm-arm/domain.h
index f3f3fb7d7f..adc7fe7210 100644
--- a/xen/include/asm-arm/domain.h
+++ b/xen/include/asm-arm/domain.h
@@ -65,9 +65,6 @@ struct arch_domain
          RELMEM_done,
      } relmem;
- struct {
-        uint64_t offset;
-    } phys_timer_base;
      struct {
          uint64_t offset;
      } virt_timer_base;


Cheers,

--
Julien Grall

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