feat(instructions): define tables::sgdt (#279)

Author: toku-sa-n

src/asm/asm.s

@@ -123,6 +123,12 @@ _x86_64_asm_lidt:
     lidt (%rdi)
     retq
 
+.global _x86_64_asm_sgdt
+.p2align 4
+_x86_64_asm_sgdt:
+    sgdt (%rdi)
+    retq
+
 .global _x86_64_asm_sidt
 .p2align 4
 _x86_64_asm_sidt:

src/asm/mod.rs

@@ -168,6 +168,12 @@ extern "sysv64" {
     )]
     pub(crate) fn x86_64_asm_lidt(idt: *const crate::instructions::tables::DescriptorTablePointer);
 
+    #[cfg_attr(
+        any(target_env = "gnu", target_env = "musl"),
+        link_name = "_x86_64_asm_sgdt"
+    )]
+    pub(crate) fn x86_64_asm_sgdt(gdt: *mut crate::instructions::tables::DescriptorTablePointer);
+
     #[cfg_attr(
         any(target_env = "gnu", target_env = "musl"),
         link_name = "_x86_64_asm_sidt"

src/instructions/tables.rs

@@ -45,6 +45,23 @@ pub unsafe fn lidt(idt: &DescriptorTablePointer) {
     crate::asm::x86_64_asm_lidt(idt as *const _);
 }
 
+/// Get the address of the current GDT.
+#[inline]
+pub fn sgdt() -> DescriptorTablePointer {
+    let mut gdt: DescriptorTablePointer = DescriptorTablePointer {
+        limit: 0,
+        base: VirtAddr::new(0),
+    };
+    unsafe {
+        #[cfg(feature = "inline_asm")]
+        asm!("sgdt [{}]", in(reg) &mut gdt, options(nostack, preserves_flags));
+
+        #[cfg(not(feature = "inline_asm"))]
+        crate::asm::x86_64_asm_sgdt(&mut gdt as *mut _);
+    }
+    gdt
+}
+
 /// Get the address of the current IDT.
 #[inline]
 pub fn sidt() -> DescriptorTablePointer {