[boot] Set up CR4 in bootloader

Moving the initial CR4 settings from the kernel's `memory_initialize`
(where it doesn't really fit anyway) to the bootloader's `hardware.cpp`.

src/boot/hardware.cpp

@@ -36,6 +36,19 @@ find_acpi_table(uefi::system_table *st)
 	return reinterpret_cast<void*>(acpi1_table);
 }
 
+void
+setup_cr4()
+{
+	uint64_t cr4 = 0;
+	asm volatile ( "mov %%cr4, %0" : "=r" (cr4) );
+	cr4 |=
+		0x000080 | // Enable global pages
+		0x000200 | // Enable FXSAVE/FXRSTOR
+		0x010000 | // Enable FSGSBASE
+		0x020000 | // Enable PCIDs
+		0;
+	asm volatile ( "mov %0, %%cr4" :: "r" (cr4) );
+}
 
 } // namespace hw
 } // namespace boot

src/boot/hardware.h

@@ -13,5 +13,8 @@ namespace hw {
 /// significant bit set to 1.
 void * find_acpi_table(uefi::system_table *st);
 
+/// Enable CPU options in CR4 for the kernel starting state.
+void setup_cr4();
+
 } // namespace hw
 } // namespace boot

src/boot/main.cpp

@@ -197,6 +197,7 @@ efi_main(uefi::handle image_handle, uefi::system_table *st)
 		L"Failed to exit boot services");
 
 	memory::virtualize(args->pml4, map, st->runtime_services);
+	hw::setup_cr4();
 
 	kentry(args);
 	debug_break();