1 files changed, 122 insertions, 0 deletions

diff --git a/arch/x86/kernel/efi_stub_32.S b/arch/x86/kernel/efi_stub_32.S
new file mode 100644
index 000000000000..ef00bb77d7e4
--- /dev/null
+++ b/arch/x86/kernel/efi_stub_32.S
@@ -0,0 +1,122 @@
+/*
+ * EFI call stub for IA32.
+ *
+ * This stub allows us to make EFI calls in physical mode with interrupts
+ * turned off.
+ */
+
+#include <linux/linkage.h>
+#include <asm/page.h>
+
+/*
+ * efi_call_phys(void *, ...) is a function with variable parameters.
+ * All the callers of this function assure that all the parameters are 4-bytes.
+ */
+
+/*
+ * In gcc calling convention, EBX, ESP, EBP, ESI and EDI are all callee save.
+ * So we'd better save all of them at the beginning of this function and restore
+ * at the end no matter how many we use, because we can not assure EFI runtime
+ * service functions will comply with gcc calling convention, too.
+ */
+
+.text
+ENTRY(efi_call_phys)
+        /*
+         * 0. The function can only be called in Linux kernel. So CS has been
+         * set to 0x0010, DS and SS have been set to 0x0018. In EFI, I found
+         * the values of these registers are the same. And, the corresponding
+         * GDT entries are identical. So I will do nothing about segment reg
+         * and GDT, but change GDT base register in prelog and epilog.
+         */
+
+        /*
+         * 1. Now I am running with EIP = <physical address> + PAGE_OFFSET.
+         * But to make it smoothly switch from virtual mode to flat mode.
+         * The mapping of lower virtual memory has been created in prelog and
+         * epilog.
+         */
+        movl    $1f, %edx
+        subl    $__PAGE_OFFSET, %edx
+        jmp     *%edx
+1:
+
+        /*
+         * 2. Now on the top of stack is the return
+         * address in the caller of efi_call_phys(), then parameter 1,
+         * parameter 2, ..., param n. To make things easy, we save the return
+         * address of efi_call_phys in a global variable.
+         */
+        popl    %edx
+        movl    %edx, saved_return_addr
+        /* get the function pointer into ECX*/
+        popl    %ecx
+        movl    %ecx, efi_rt_function_ptr
+        movl    $2f, %edx
+        subl    $__PAGE_OFFSET, %edx
+        pushl   %edx
+
+        /*
+         * 3. Clear PG bit in %CR0.
+         */
+        movl    %cr0, %edx
+        andl    $0x7fffffff, %edx
+        movl    %edx, %cr0
+        jmp     1f
+1:
+
+        /*
+         * 4. Adjust stack pointer.
+         */
+        subl    $__PAGE_OFFSET, %esp
+
+        /*
+         * 5. Call the physical function.
+         */
+        jmp     *%ecx
+
+2:
+        /*
+         * 6. After EFI runtime service returns, control will return to
+         * following instruction. We'd better readjust stack pointer first.
+         */
+        addl    $__PAGE_OFFSET, %esp
+
+        /*
+         * 7. Restore PG bit
+         */
+        movl    %cr0, %edx
+        orl     $0x80000000, %edx
+        movl    %edx, %cr0
+        jmp     1f
+1:
+        /*
+         * 8. Now restore the virtual mode from flat mode by
+         * adding EIP with PAGE_OFFSET.
+         */
+        movl    $1f, %edx
+        jmp     *%edx
+1:
+
+        /*
+         * 9. Balance the stack. And because EAX contain the return value,
+         * we'd better not clobber it.
+         */
+        leal    efi_rt_function_ptr, %edx
+        movl    (%edx), %ecx
+        pushl   %ecx
+
+        /*
+         * 10. Push the saved return address onto the stack and return.
+         */
+        leal    saved_return_addr, %edx
+        movl    (%edx), %ecx
+        pushl   %ecx
+        ret
+.previous
+
+.data
+saved_return_addr:
+        .long 0
+efi_rt_function_ptr:
+        .long 0