x86/efi: Add early thunk code to go from 64-bit to 32-bit

Implement the transition code to go from IA32e mode to protected mode in
the EFI boot stub. This is required to use 32-bit EFI services from a
64-bit kernel.

Since EFI boot stub is executed in an identity-mapped region, there's
not much we need to do before invoking the 32-bit EFI boot services.
However, we do reload the firmware's global descriptor table
(efi32_boot_gdt) in case things like timer events are still running in
the firmware.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 150 insertions, 0 deletions

diff --git a/arch/x86/platform/efi/efi_stub_64.S b/arch/x86/platform/efi/efi_stub_64.S
index 88073b140298..a790d69cc85e 100644
--- a/arch/x86/platform/efi/efi_stub_64.S
+++ b/arch/x86/platform/efi/efi_stub_64.S
@@ -7,6 +7,10 @@
  */
 
 #include <linux/linkage.h>
+#include <asm/segment.h>
+#include <asm/msr.h>
+#include <asm/processor-flags.h>
+#include <asm/page_types.h>
 
 #define SAVE_XMM                        \
         mov %rsp, %rax;                 \
@@ -164,6 +168,152 @@ ENTRY(efi_call6)
         ret
 ENDPROC(efi_call6)
 
+#ifdef CONFIG_EFI_MIXED
+
+/*
+ * We run this function from the 1:1 mapping.
+ *
+ * This function must be invoked with a 1:1 mapped stack.
+ */
+ENTRY(__efi64_thunk)
+        subq    $32, %rsp
+        movl    %esi, 0x0(%rsp)
+        movl    %edx, 0x4(%rsp)
+        movl    %ecx, 0x8(%rsp)
+        movq    %r8, %rsi
+        movl    %esi, 0xc(%rsp)
+        movq    %r9, %rsi
+        movl    %esi,  0x10(%rsp)
+
+        sgdt    save_gdt(%rip)
+
+        leaq    1f(%rip), %rbx
+        movq    %rbx, func_rt_ptr(%rip)
+
+        /* Switch to gdt with 32-bit segments */
+        movl    40(%rsp), %eax
+        lgdt    (%rax)
+
+        leaq    efi_enter32(%rip), %rax
+        pushq   $__KERNEL_CS
+        pushq   %rax
+        lretq
+
+1:      addq    $32, %rsp
+
+        lgdt    save_gdt(%rip)
+
+        /*
+         * Convert 32-bit status code into 64-bit.
+         */
+        test    %rax, %rax
+        jz      1f
+        movl    %eax, %ecx
+        andl    $0x0fffffff, %ecx
+        andl    $0xf0000000, %eax
+        shl     $32, %rax
+        or      %rcx, %rax
+1:
+        ret
+ENDPROC(__efi64_thunk)
+
+ENTRY(efi_exit32)
+        xorq    %rax, %rax
+        movl    %eax, %ds
+        movl    %eax, %es
+        movl    %eax, %ss
+
+        movq    func_rt_ptr(%rip), %rax
+        push    %rax
+        mov     %rdi, %rax
+        ret
+ENDPROC(efi_exit32)
+
+        .code32
+/*
+ * EFI service pointer must be in %edi.
+ *
+ * The stack should represent the 32-bit calling convention.
+ */
+ENTRY(efi_enter32)
+        movl    $__KERNEL_DS, %eax
+        movl    %eax, %ds
+        movl    %eax, %es
+        movl    %eax, %ss
+
+        /* Reload pgtables */
+        movl    %cr3, %eax
+        movl    %eax, %cr3
+
+        /* Disable paging */
+        movl    %cr0, %eax
+        btrl    $X86_CR0_PG_BIT, %eax
+        movl    %eax, %cr0
+
+        /* Disable long mode via EFER */
+        movl    $MSR_EFER, %ecx
+        rdmsr
+        btrl    $_EFER_LME, %eax
+        wrmsr
+
+        call    *%edi
+
+        /* We must preserve return value */
+        movl    %eax, %edi
+
+        movl    44(%esp), %eax
+        movl    %eax, 2(%eax)
+        lgdtl   (%eax)
+
+        movl    %cr4, %eax
+        btsl    $(X86_CR4_PAE_BIT), %eax
+        movl    %eax, %cr4
+
+        movl    %cr3, %eax
+        movl    %eax, %cr3
+
+        movl    $MSR_EFER, %ecx
+        rdmsr
+        btsl    $_EFER_LME, %eax
+        wrmsr
+
+        xorl    %eax, %eax
+        lldt    %ax
+
+        movl    48(%esp), %eax
+        pushl   $__KERNEL_CS
+        pushl   %eax
+
+        /* Enable paging */
+        movl    %cr0, %eax
+        btsl    $X86_CR0_PG_BIT, %eax
+        movl    %eax, %cr0
+        lret
+ENDPROC(efi_enter32)
+
+        .data
+        .balign 8
+        .global efi32_boot_gdt
+efi32_boot_gdt: .word   0
+                .quad   0
+
+save_gdt:       .word   0
+                .quad   0
+func_rt_ptr:    .quad   0
+
+        .global efi_gdt64
+efi_gdt64:
+        .word   efi_gdt64_end - efi_gdt64
+        .long   0                       /* Filled out by user */
+        .word   0
+        .quad   0x0000000000000000      /* NULL descriptor */
+        .quad   0x00af9a000000ffff      /* __KERNEL_CS */
+        .quad   0x00cf92000000ffff      /* __KERNEL_DS */
+        .quad   0x0080890000000000      /* TS descriptor */
+        .quad   0x0000000000000000      /* TS continued */
+efi_gdt64_end:
+#endif /* CONFIG_EFI_MIXED */
+
         .data
 ENTRY(efi_scratch)
         .fill 3,8,0