[PATCH v4 01/21] x86/efistub: Branch straight to kernel entry point from C code

[Ard Biesheuvel <ardb@kernel.org>]

Instead of returning to the calling code in assembler that does nothing
more than perform an indirect call with the boot_params pointer in
register ESI/RSI, perform the jump directly from the EFI stub C code.
This will allow the asm entrypoint code to be dropped entirely in
subsequent patches.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/x86-stub.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index 8d17cee8b98e1a63..095aaa8b0ee30fb9 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -769,9 +769,17 @@ static efi_status_t exit_boot(struct boot_params *boot_params, void *handle)
 	return EFI_SUCCESS;
 }
 
+static void __noreturn enter_kernel(unsigned long kernel_addr,
+				    struct boot_params *boot_params)
+{
+	/* enter decompressed kernel with boot_params pointer in RSI/ESI */
+	asm("jmp *%0"::"r"(kernel_addr), "S"(boot_params));
+
+	unreachable();
+}
+
 /*
- * On success, we return the address of startup_32, which has potentially been
- * relocated by efi_relocate_kernel.
+ * On success, we jump to the relocated kernel directly and never return.
  * On failure, we exit to the firmware via efi_exit instead of returning.
  */
 asmlinkage unsigned long efi_main(efi_handle_t handle,
@@ -914,7 +922,11 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
 		goto fail;
 	}
 
-	return bzimage_addr;
+	if (IS_ENABLED(CONFIG_X86_64))
+		/* add offset of startup_64() */
+		bzimage_addr += 0x200;
+
+	enter_kernel(bzimage_addr, boot_params);
 fail:
 	efi_err("efi_main() failed!\n");
 
-- 
2.39.2