x86_64 isr: don't rely on the value of rbp

src/arch/x86_64/isr.asm

@@ -59,11 +59,6 @@ section .text
 	
 	add rsp, 24 ; cr2 gs fs
 	
-	cmp rbp,0x3b
-	jne .noswapgspop
-	swapgs
-	.noswapgspop:
-	
 	pop rax
 	mov es,rax
 	pop rax
@@ -85,6 +80,13 @@ section .text
 	pop rbp
 	add rsp,8 ; remove error code
 
+	cmp qword [rsp+8], 0x3b ; user cs
+	jne .noswap
+
+	swapgs
+
+	.noswap:
+
 %endmacro
 
 %macro isr 2