real_mode: Fix long to protected mode transition

Going into real mode would crash when clearing EFER_LME, i.e., when transitioning from long to protected mode. This happened because paging was still enabled. Before disabling paging, we must also set the %ds. Signed-off-by: Johannes Wikner <[email protected]>
KernelTestFramework · Aug 29, 2023 · 06a81f4 · 06a81f4
1 parent a5164e3
commit 06a81f4
Showing 1 changed file with 12 additions and 10 deletions.
diff --git a/arch/x86/real_mode.S b/arch/x86/real_mode.S
@@ -139,7 +139,18 @@ END_FUNC(_prot_to_real)
 
 .align 16
 .Lfrom_long_mode:
-    /* Disable LME in EFER */
+    /* Use protected mode data segment */
+    mov   $__KERN_DS32, %eax
+    mov   %eax, %ds
+    mov   %eax, %ss
+
+    /* Disable paging to enter protected mode. */
+    /* Clearing PG implicitly clears EFER.LMA, see Intel SDM 9.8.5.4, */
+    mov   %cr0, %eax
+    and   $~(X86_CR0_PG | X86_CR0_WP), %eax
+    mov   %eax, %cr0
+
+    /* Disable LME in EFER. */
     movl  $MSR_EFER, %ecx
     rdmsr
     and   $~EFER_LME, %eax
@@ -151,15 +162,6 @@ END_FUNC(_prot_to_real)
     lgdt  rmode_gdt_ptr
     lidt  rmode_idt_ptr
 
-    /* Disable paging to enter protected mode */
-    mov   %cr0, %eax
-    and   $~(X86_CR0_PG | X86_CR0_WP), %eax
-    mov   %eax, %cr0
-
-    /* Use protected mode data selector */
-    mov   $__KERN_DS32, %eax
-    mov   %eax, %ds
-
     /* Use real mode accessible stack */
     mov   rmode_stack, %esp