[PATCH v2 4/8] x86/IRQ: avoid over-alignment in alloc_pirq_struct()

[Jan Beulich <jbeulich@xxxxxxxx>]

In particular in the PV case xzalloc_bytes() forcing SMP_CACHE_BYTES
alignment is counterproductive, as the allocation size there is only 40
bytes. And if the code really cared about such higher than default
alignment, it should request so explicitly rather than using a type-
unsafe interface. Plus if e.g. cache line sharing was a concern, the
allocator itself should arrange to avoid such.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -1313,9 +1313,12 @@ void cleanup_domain_irq_mapping(struct d
 
 struct pirq *alloc_pirq_struct(struct domain *d)
 {
-    size_t sz = is_hvm_domain(d) ? sizeof(struct pirq) :
-                                   offsetof(struct pirq, arch.hvm);
-    struct pirq *pirq = xzalloc_bytes(sz);
+    union pirq_pv {
+        char space[offsetof(struct pirq, arch.hvm)];
+        void *align;
+    };
+    struct pirq *pirq = is_hvm_domain(d) ? xzalloc(struct pirq)
+                                         : (void *)xzalloc(union pirq_pv);
 
     if ( pirq )
     {