diff --git a/src/arch/x86/acpi.c b/src/arch/x86/acpi.c
index a21bae2a36..5640ad0654 100644
--- a/src/arch/x86/acpi.c
+++ b/src/arch/x86/acpi.c
@@ -411,7 +411,7 @@ void acpi_create_dmar(acpi_dmar_t *dmar, enum dmar_flags flags,
 	header->length = sizeof(acpi_dmar_t);
 	header->revision = 1;
 
-	dmar->host_address_width = 40 - 1; /* FIXME: == MTRR size? */
+	dmar->host_address_width = cpu_phys_address_size() - 1;
 	dmar->flags = flags;
 
 	current = acpi_fill_dmar(current);