xen/sched: we never get into context_switch() with prev==next

In schedule(), if we pick, as the next vcpu to run (next) the same one
that is running already (prev), we never get to call context_switch().

We can, therefore, get rid of all the `if`-s testing prev and next being
different, trading them with an ASSERT() (on ARM, the ASSERT() was even
already there!)

Suggested-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>

xen/arch/arm/domain.c

@@ -343,8 +343,7 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
     ASSERT(prev != next);
     ASSERT(!vcpu_cpu_dirty(next));
 
-    if ( prev != next )
-        update_runstate_area(prev);
+    update_runstate_area(prev);
 
     local_irq_disable();
 

xen/arch/x86/domain.c

@@ -1721,6 +1721,7 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
     const struct domain *prevd = prev->domain, *nextd = next->domain;
     unsigned int dirty_cpu = next->dirty_cpu;
 
+    ASSERT(prev != next);
     ASSERT(local_irq_is_enabled());
 
     get_cpu_info()->use_pv_cr3 = false;
@@ -1732,12 +1733,9 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
         flush_mask(cpumask_of(dirty_cpu), FLUSH_VCPU_STATE);
     }
 
-    if ( prev != next )
-    {
-        _update_runstate_area(prev);
-        vpmu_switch_from(prev);
-        np2m_schedule(NP2M_SCHEDLE_OUT);
-    }
+    _update_runstate_area(prev);
+    vpmu_switch_from(prev);
+    np2m_schedule(NP2M_SCHEDLE_OUT);
 
     if ( is_hvm_domain(prevd) && !list_empty(&prev->arch.hvm.tm_list) )
         pt_save_timer(prev);
@@ -1794,14 +1792,10 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
 
     context_saved(prev);
 
-    if ( prev != next )
-    {
-        _update_runstate_area(next);
-
-        /* Must be done with interrupts enabled */
-        vpmu_switch_to(next);
-        np2m_schedule(NP2M_SCHEDLE_IN);
-    }
+    _update_runstate_area(next);
+    /* Must be done with interrupts enabled */
+    vpmu_switch_to(next);
+    np2m_schedule(NP2M_SCHEDLE_IN);
 
     /* Ensure that the vcpu has an up-to-date time base. */
     update_vcpu_system_time(next);