x86/efi/mixed: Simplify and document thunking logic

Now that the GDT/IDT and data segment selector preserve/restore logic
has been removed from the boot-time EFI mixed mode thunking routines,
the remaining logic to handle the function arguments can be simplified:
the setup of the arguments on the stack can be moved into the 32-bit
callee, which is able to use a more idiomatic sequence of PUSH
instructions.

This, in turn, allows the far call and far return to be issued using
plain LCALL and LRET instructions, removing the need to set up the
return explicitly.

Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S
index 984956931ed759383eb16d82f391c1f763227e1f..e04ed99bc449e72e2367f890742d5a8b517f6b0d 100644 (file)
--- a/arch/x86/boot/compressed/efi_mixed.S
+++ b/arch/x86/boot/compressed/efi_mixed.S
@@ -22,43 +22,7 @@
 #include <asm/processor-flags.h>
 #include <asm/segment.h>
 
-       .code64
        .text
-SYM_FUNC_START(__efi64_thunk)
-       push    %rbp
-       push    %rbx
-
-       /* Copy args passed on stack */
-       movq    0x18(%rsp), %rbp
-       movq    0x20(%rsp), %rbx
-       movq    0x28(%rsp), %rax
-
-       /*
-        * Convert x86-64 ABI params to i386 ABI
-        */
-       subq    $64, %rsp
-       movl    %esi, 0x0(%rsp)
-       movl    %edx, 0x4(%rsp)
-       movl    %ecx, 0x8(%rsp)
-       movl    %r8d, 0xc(%rsp)
-       movl    %r9d, 0x10(%rsp)
-       movl    %ebp, 0x14(%rsp)
-       movl    %ebx, 0x18(%rsp)
-       movl    %eax, 0x1c(%rsp)
-
-       leaq    1f(%rip), %rbp
-       movl    %cs, %ebx
-
-       ljmpl   *efi32_call(%rip)
-
-1:     addq    $64, %rsp
-       movq    %rdi, %rax
-
-       pop     %rbx
-       pop     %rbp
-       RET
-SYM_FUNC_END(__efi64_thunk)
-
        .code32
 #ifdef CONFIG_EFI_HANDOVER_PROTOCOL
 SYM_FUNC_START(efi32_stub_entry)
@@ -81,11 +45,26 @@ SYM_FUNC_END(efi32_stub_entry)
 #endif
 
 /*
- * EFI service pointer must be in %edi.
+ * Called using a far call from __efi64_thunk() below, using the x86_64 SysV
+ * ABI (except for R8/R9 which are inaccessible to 32-bit code - EAX/EBX are
+ * used instead).  EBP+16 points to the arguments passed via the stack.
  *
- * The stack should represent the 32-bit calling convention.
+ * The first argument (EDI) is a pointer to the boot service or protocol, to
+ * which the remaining arguments are passed, each truncated to 32 bits.
  */
 SYM_FUNC_START_LOCAL(efi_enter32)
+       /*
+        * Convert x86-64 SysV ABI params to i386 ABI
+        */
+       pushl   32(%ebp)        /* Up to 3 args passed via the stack */
+       pushl   24(%ebp)
+       pushl   16(%ebp)
+       pushl   %ebx            /* R9 */
+       pushl   %eax            /* R8 */
+       pushl   %ecx
+       pushl   %edx
+       pushl   %esi
+
        /* Disable paging */
        movl    %cr0, %eax
        btrl    $X86_CR0_PG_BIT, %eax
@@ -104,11 +83,29 @@ SYM_FUNC_START_LOCAL(efi_enter32)
 
        call    efi32_enable_long_mode
 
-       pushl   %ebx
-       pushl   %ebp
+       addl    $32, %esp
+       movl    %edi, %eax
        lret
 SYM_FUNC_END(efi_enter32)
 
+       .code64
+SYM_FUNC_START(__efi64_thunk)
+       push    %rbp
+       movl    %esp, %ebp
+       push    %rbx
+
+       /* Move args #5 and #6 into 32-bit accessible registers */
+       movl    %r8d, %eax
+       movl    %r9d, %ebx
+
+       lcalll  *efi32_call(%rip)
+
+       pop     %rbx
+       pop     %rbp
+       RET
+SYM_FUNC_END(__efi64_thunk)
+
+       .code32
 SYM_FUNC_START_LOCAL(efi32_enable_long_mode)
        movl    %cr4, %eax
        btsl    $(X86_CR4_PAE_BIT), %eax