x86: reserve the first megabyte

A more comprehensive solution would use E820 enumeration, but we
are unlikely to ever care that much, as we intend to use demand
paging on microcontrollers and not PC-like hardware. This is
really to just prevent QEMU from crashing.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

arch/x86/core/x86_mmu.c

@@ -17,6 +17,7 @@
 #include <x86_mmu.h>
 #include <init.h>
 #include <kernel_internal.h>
+#include <mmu.h>
 #include <drivers/interrupt_controller/loapic.h>
 
 LOG_MODULE_DECLARE(os, CONFIG_KERNEL_LOG_LEVEL);
@@ -1775,3 +1776,22 @@ void z_x86_current_stack_perms(void)
 #endif
 }
 #endif /* CONFIG_USERSPACE */
+
+#ifdef CONFIG_ARCH_HAS_RESERVED_PAGE_FRAMES
+/* Selected on PC-like targets at the SOC level.
+ *
+ * Best is to do some E820 or similar enumeration to specifically identify
+ * all page frames which are reserved by the hardware or firmware.
+ *
+ * For now, just reserve everything in the first megabyte of physical memory.
+ */
+void arch_reserved_pages_update(void)
+{
+	for (uintptr_t pos = 0; pos < (1024 * 1024);
+	     pos += CONFIG_MMU_PAGE_SIZE) {
+		struct z_page_frame *pf = z_phys_to_page_frame(pos);
+
+		pf->flags |= Z_PAGE_FRAME_RESERVED;
+	}
+}
+#endif /* CONFIG_ARCH_HAS_RESERVED_PAGE_FRAMES */

soc/x86/apollo_lake/Kconfig.soc

@@ -10,3 +10,4 @@ config SOC_APOLLO_LAKE
 	select PCIE_MSI
 	select DYNAMIC_INTERRUPTS
 	select X86_MMU
+	select ARCH_HAS_RESERVED_PAGE_FRAMES

soc/x86/atom/Kconfig.soc

@@ -6,3 +6,4 @@ config SOC_ATOM
 	select CPU_ATOM
 	select X86_MMU
 	select ARCH_HAS_USERSPACE
+	select ARCH_HAS_RESERVED_PAGE_FRAMES

soc/x86/ia32/Kconfig.soc

@@ -4,3 +4,4 @@ config SOC_IA32
 	bool "Generic IA32 SoC"
 	select X86
 	select CPU_MINUTEIA
+	select ARCH_HAS_RESERVED_PAGE_FRAMES if SRAM_BASE_ADDRESS = 0