executor: introduce global state page for SYZOS

Reserve a dedicated 4KB page at X86_SYZOS_ADDR_GLOBALS (0x17F000) to
store global state shared across the SYZOS L1 guest environment.

This region is required to store the state of the guest-side memory
allocator (specifically the allocation offset and total size of the
unused heap), enabling thread-safe dynamic memory allocation for nested
L2 page tables.

Author: ramosian-glider

executor/common_kvm_amd64.h

@@ -237,6 +237,8 @@ static const struct mem_region syzos_mem_regions[] = {
     {X86_SYZOS_ADDR_BOOT_ARGS, 1, 0},
     // Dynamic Page Table Pool.
     {X86_SYZOS_ADDR_PT_POOL, X86_SYZOS_PT_POOL_SIZE, 0},
+    // Global State Page.
+    {X86_SYZOS_ADDR_GLOBALS, 1, 0},
     // SMRAM memory.
     {X86_SYZOS_ADDR_SMRAM, 10, 0},
     // Unmapped region to trigger a page faults for uexits etc.

executor/kvm.h

@@ -76,6 +76,9 @@
 // Base offset for the area containing the 4 L2 VM slots.
 #define X86_SYZOS_L1_VCPU_OFFSET_L2_VMS_AREA 0x1000
 
+// Global state page (Allocator offsets, etc).
+#define X86_SYZOS_ADDR_GLOBALS 0x17F000
+
 // Separated Page Table Pool in high memory.
 // Located above L2 VCPU regions.
 #define X86_SYZOS_ADDR_PT_POOL 0x180000