[Xen-devel] [RFC 14/16] hack: arm/domain: simplify context restore from idle vcpu

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

From: Andrii Anisov <andrii_anisov@xxxxxxxx>

Simplify context restore from idle vcpu to the one ran before it.
This improves low cpu load but high irq rate use-cases.

Signed-off-by: Andrii Anisov <andrii_anisov@xxxxxxxx>
---
 xen/arch/arm/domain.c   | 21 +++++++++++----------
 xen/include/xen/sched.h |  1 +
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 1d926dc..8e886b7 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -187,9 +187,6 @@ static void ctxt_switch_to(struct vcpu *n)
     WRITE_SYSREG32(vpidr, VPIDR_EL2);
     WRITE_SYSREG(n->arch.vmpidr, VMPIDR_EL2);
 
-    /* VGIC */
-    gic_restore_state(n);
-
     /* VFP */
     vfp_restore_state(n);
 
@@ -263,11 +260,6 @@ static void ctxt_switch_to(struct vcpu *n)
     WRITE_SYSREG(n->arch.csselr, CSSELR_EL1);
 
     isb();
-
-    /* This is could trigger an hardware interrupt from the virtual
-     * timer. The interrupt needs to be injected into the guest. */
-    WRITE_SYSREG32(n->arch.cntkctl, CNTKCTL_EL1);
-    virt_timer_restore(n);
 }
 
 /* Update per-VCPU guest runstate shared memory area (if registered). */
@@ -302,8 +294,17 @@ static void update_runstate_area(struct vcpu *v)
 static void schedule_tail(struct vcpu *prev)
 {
     ctxt_switch_from(prev);
-
-    ctxt_switch_to(current);
+    if ( !(is_idle_vcpu(prev) && (prev->prev == current)) )
+        ctxt_switch_to(current);
+    /* VGIC */
+    if ( !is_idle_vcpu(current) )
+    {
+        gic_restore_state(current);
+        WRITE_SYSREG32(current->arch.cntkctl, CNTKCTL_EL1);
+        virt_timer_restore(current);
+    }
+    else
+        current->prev = prev;
 
     local_irq_enable();
 
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 0309c1f..e85108d 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -272,6 +272,7 @@ struct vcpu
     struct vpci_vcpu vpci;
 
     struct arch_vcpu arch;
+    struct vcpu *prev;
 };
 
 /* Per-domain lock can be recursively acquired in fault handlers. */
-- 
2.7.4


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