nested-svm-vmrun: document and name GDT descriptor size checks

Agent-Logs-Url: https://github.com/bernhard-xs/xtf/sessions/ff7f3b83-f4cf-44a0-be6e-69decc50df9f

Co-authored-by: bernhardkaindl <43588962+bernhardkaindl@users.noreply.github.com>

Author: Copilot

tests/nested-svm-vmrun/main.c

@@ -41,6 +41,8 @@ void svm_vmrun(unsigned long l2_vmcb_pa);
 #define L2_SENTINEL 0xc0ffeeULL
 static volatile uint64_t l2_handshake;
 
+#define GDT_DESC_BYTES 8
+
 static uint16_t user_desc_vmcb_attr(const user_desc *desc)
 {
     return desc->type |
@@ -63,7 +65,6 @@ static bool selector_is_null(uint16_t sel)
 static void vmcb_set_seg_desc(struct vmcb_seg *seg, const user_desc *gdt,
                               uint16_t gdt_limit, uint16_t sel)
 {
-    const uint16_t desc_size = 8;
     uint16_t sel_offset = sel & ~(X86_SEL_TI | X86_SEL_RPL_MASK);
     const user_desc *desc;
 
@@ -77,8 +78,9 @@ static void vmcb_set_seg_desc(struct vmcb_seg *seg, const user_desc *gdt,
         return;
     }
 
+    /* Verify the descriptor's last byte still lies within the GDT limit. */
     if ( (sel & X86_SEL_TI) ||
-         (sel_offset + desc_size - 1 > gdt_limit) )
+         (sel_offset + GDT_DESC_BYTES - 1 > gdt_limit) )
     {
         seg->attr = 0;
         seg->limit = 0;