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Diffstat (limited to 'arch/x86/boot/compressed/efi_stub_32.S')
-rw-r--r--arch/x86/boot/compressed/efi_stub_32.S86
1 files changed, 86 insertions, 0 deletions
diff --git a/arch/x86/boot/compressed/efi_stub_32.S b/arch/x86/boot/compressed/efi_stub_32.S
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index 000000000000..a53440e81d52
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+++ b/arch/x86/boot/compressed/efi_stub_32.S
@@ -0,0 +1,86 @@
+/*
+ * EFI call stub for IA32.
+ *
+ * This stub allows us to make EFI calls in physical mode with interrupts
+ * turned off. Note that this implementation is different from the one in
+ * arch/x86/platform/efi/efi_stub_32.S because we're _already_ in physical
+ * mode at this point.
+ */
+
+#include <linux/linkage.h>
+#include <asm/page_types.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. Because we haven't been relocated by this point we need to
+ * use relative addressing.
+ */
+ call 1f
+1: popl %edx
+ subl $1b, %edx
+
+ /*
+ * 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 %ecx
+ movl %ecx, saved_return_addr(%edx)
+ /* get the function pointer into ECX*/
+ popl %ecx
+ movl %ecx, efi_rt_function_ptr(%edx)
+
+ /*
+ * 3. Call the physical function.
+ */
+ call *%ecx
+
+ /*
+ * 4. Balance the stack. And because EAX contain the return value,
+ * we'd better not clobber it. We need to calculate our address
+ * again because %ecx and %edx are not preserved across EFI function
+ * calls.
+ */
+ call 1f
+1: popl %edx
+ subl $1b, %edx
+
+ movl efi_rt_function_ptr(%edx), %ecx
+ pushl %ecx
+
+ /*
+ * 10. Push the saved return address onto the stack and return.
+ */
+ movl saved_return_addr(%edx), %ecx
+ pushl %ecx
+ ret
+ENDPROC(efi_call_phys)
+.previous
+
+.data
+saved_return_addr:
+ .long 0
+efi_rt_function_ptr:
+ .long 0