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Message-ID: <20250210174941.3251435-14-ardb+git@google.com> Date: Mon, 10 Feb 2025 18:49:47 +0100 From: Ard Biesheuvel <ardb+git@...gle.com> To: linux-efi@...r.kernel.org Cc: linux-kernel@...r.kernel.org, x86@...nel.org, hdegoede@...hat.com, Ard Biesheuvel <ardb@...nel.org> Subject: [PATCH v2 5/7] x86/efi/mixed: Remove dependency on legacy startup_32 code From: Ard Biesheuvel <ardb@...nel.org> The EFI mixed mode startup code calls into startup_32() in the legacy decompressor, passing a mocked up boot_params struct, only to get it to set up the 1:1 mapping of the lower 4 GiB of memory and switch to a GDT that supports 64-bit mode. In order to be able to reuse the EFI mixed mode startup code in EFI boot images that do not incorporate the legacy decompressor code, decouple it, by populating the GDT directly. Doing so allows constructing a GDT that is compatible with the one the firmware uses, with one additional entry for a 64-bit mode code segment appended. This removes the need entirely to switch between GDTs and IDTs or data segment selector values on every call into the firmware, and all of this code can be removed. Signed-off-by: Ard Biesheuvel <ardb@...nel.org> --- arch/x86/boot/compressed/efi_mixed.S | 227 ++++++-------------- arch/x86/boot/compressed/head_64.S | 7 - 2 files changed, 65 insertions(+), 169 deletions(-) diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S index dca916c3e6f0..984956931ed7 100644 --- a/arch/x86/boot/compressed/efi_mixed.S +++ b/arch/x86/boot/compressed/efi_mixed.S @@ -15,70 +15,23 @@ */ #include <linux/linkage.h> -#include <asm/asm-offsets.h> +#include <asm/desc_defs.h> #include <asm/msr.h> #include <asm/page_types.h> #include <asm/pgtable_types.h> #include <asm/processor-flags.h> #include <asm/segment.h> -#include <asm/setup.h> .code64 .text -/* - * When booting in 64-bit mode on 32-bit EFI firmware, startup_64_mixed_mode() - * is the first thing that runs after switching to long mode. Depending on - * whether the EFI handover protocol or the compat entry point was used to - * enter the kernel, it will either branch to the common 64-bit EFI stub - * entrypoint efi_stub_entry() directly, or via the 64-bit EFI PE/COFF - * entrypoint efi_pe_entry(). In the former case, the bootloader must provide a - * struct bootparams pointer as the third argument, so the presence of such a - * pointer is used to disambiguate. - * - * +--------------+ - * +------------------+ +------------+ +------>| efi_pe_entry | - * | efi32_pe_entry |---->| | | +-----------+--+ - * +------------------+ | | +------+----------------+ | - * | startup_32 |---->| startup_64_mixed_mode | | - * +------------------+ | | +------+----------------+ | - * | efi32_stub_entry |---->| | | | - * +------------------+ +------------+ | | - * V | - * +------------+ +----------------+ | - * | startup_64 |<----| efi_stub_entry |<--------+ - * +------------+ +----------------+ - */ -SYM_FUNC_START(startup_64_mixed_mode) - lea efi32_boot_args(%rip), %rdx - mov 0(%rdx), %edi - mov 4(%rdx), %esi - - leaq (pte + 5 * PAGE_SIZE)(%rip), %rax - movq %rax, %cr3 // reload after startup_32 - - /* Switch to the firmware's stack */ - movl efi32_boot_sp(%rip), %esp - andl $~7, %esp - - mov 8(%rdx), %edx // saved bootparams pointer - call efi_stub_entry -SYM_FUNC_END(startup_64_mixed_mode) - SYM_FUNC_START(__efi64_thunk) push %rbp push %rbx - movl %ds, %eax - push %rax - movl %es, %eax - push %rax - movl %ss, %eax - push %rax - /* Copy args passed on stack */ - movq 0x30(%rsp), %rbp - movq 0x38(%rsp), %rbx - movq 0x40(%rsp), %rax + movq 0x18(%rsp), %rbp + movq 0x20(%rsp), %rbx + movq 0x28(%rsp), %rax /* * Convert x86-64 ABI params to i386 ABI @@ -93,44 +46,14 @@ SYM_FUNC_START(__efi64_thunk) movl %ebx, 0x18(%rsp) movl %eax, 0x1c(%rsp) - leaq 0x20(%rsp), %rbx - sgdt (%rbx) - sidt 16(%rbx) - leaq 1f(%rip), %rbp + movl %cs, %ebx - /* - * Switch to IDT and GDT with 32-bit segments. These are the firmware - * GDT and IDT that were installed when the kernel started executing. - * The pointers were saved by the efi32_entry() routine below. - * - * Pass the saved DS selector to the 32-bit code, and use far return to - * restore the saved CS selector. - */ - lidt efi32_boot_idt(%rip) - lgdt efi32_boot_gdt(%rip) - - movzwl efi32_boot_ds(%rip), %edx - movzwq efi32_boot_cs(%rip), %rax - pushq %rax - leaq efi_enter32(%rip), %rax - pushq %rax - lretq + ljmpl *efi32_call(%rip) 1: addq $64, %rsp movq %rdi, %rax - pop %rbx - movl %ebx, %ss - pop %rbx - movl %ebx, %es - pop %rbx - movl %ebx, %ds - /* Clear out 32-bit selector from FS and GS */ - xorl %ebx, %ebx - movl %ebx, %fs - movl %ebx, %gs - pop %rbx pop %rbp RET @@ -141,7 +64,6 @@ SYM_FUNC_END(__efi64_thunk) SYM_FUNC_START(efi32_stub_entry) call 1f 1: popl %ecx - leal (efi32_boot_args - 1b)(%ecx), %ebx /* Clear BSS */ xorl %eax, %eax @@ -153,11 +75,8 @@ SYM_FUNC_START(efi32_stub_entry) rep stosl add $0x4, %esp /* Discard return address */ - popl %ecx - popl %edx - popl %esi - movl %esi, 8(%ebx) - jmp efi32_entry + movl 8(%esp), %ebx /* struct boot_params pointer */ + jmp efi32_startup SYM_FUNC_END(efi32_stub_entry) #endif @@ -167,13 +86,6 @@ SYM_FUNC_END(efi32_stub_entry) * The stack should represent the 32-bit calling convention. */ SYM_FUNC_START_LOCAL(efi_enter32) - /* Load firmware selector into data and stack segment registers */ - movl %edx, %ds - movl %edx, %es - movl %edx, %fs - movl %edx, %gs - movl %edx, %ss - /* Disable paging */ movl %cr0, %eax btrl $X86_CR0_PG_BIT, %eax @@ -190,21 +102,9 @@ SYM_FUNC_START_LOCAL(efi_enter32) /* We must preserve return value */ movl %eax, %edi - /* - * Some firmware will return with interrupts enabled. Be sure to - * disable them before we switch GDTs and IDTs. - */ - cli - - lidtl 16(%ebx) - lgdtl (%ebx) - - xorl %eax, %eax - lldt %ax - call efi32_enable_long_mode - pushl $__KERNEL_CS + pushl %ebx pushl %ebp lret SYM_FUNC_END(efi_enter32) @@ -230,50 +130,56 @@ SYM_FUNC_START_LOCAL(efi32_enable_long_mode) SYM_FUNC_END(efi32_enable_long_mode) /* - * This is the common EFI stub entry point for mixed mode. + * This is the common EFI stub entry point for mixed mode. It sets up the GDT + * and page tables needed for 64-bit execution, after which it calls the + * common 64-bit EFI entrypoint efi_stub_entry(). * - * Arguments: %ecx image handle - * %edx EFI system table pointer + * Arguments: 0(%esp) image handle + * 4(%esp) EFI system table pointer + * %ebx struct boot_params pointer (or NULL) * * Since this is the point of no return for ordinary execution, no registers * are considered live except for the function parameters. [Note that the EFI * stub may still exit and return to the firmware using the Exit() EFI boot * service.] */ -SYM_FUNC_START_LOCAL(efi32_entry) - call 1f -1: pop %ebx - - /* Save firmware GDTR and code/data selectors */ - sgdtl (efi32_boot_gdt - 1b)(%ebx) - movw %cs, (efi32_boot_cs - 1b)(%ebx) - movw %ds, (efi32_boot_ds - 1b)(%ebx) - - /* Store firmware IDT descriptor */ - sidtl (efi32_boot_idt - 1b)(%ebx) - - /* Store firmware stack pointer */ - movl %esp, (efi32_boot_sp - 1b)(%ebx) - - /* Store boot arguments */ - leal (efi32_boot_args - 1b)(%ebx), %ebx - movl %ecx, 0(%ebx) - movl %edx, 4(%ebx) - movb $0x0, 12(%ebx) // efi_is64 - - /* - * Allocate some memory for a temporary struct boot_params, which only - * needs the minimal pieces that startup_32() relies on. - */ - subl $PARAM_SIZE, %esp - movl %esp, %esi - movl $PAGE_SIZE, BP_kernel_alignment(%esi) - movl $_end - 1b, BP_init_size(%esi) - subl $startup_32 - 1b, BP_init_size(%esi) +SYM_FUNC_START_LOCAL(efi32_startup) + movl %esp, %ebp + + subl $8, %esp + sgdtl (%esp) /* Save GDT descriptor to the stack */ + movl 2(%esp), %esi /* Existing GDT pointer */ + movzwl (%esp), %ecx /* Existing GDT limit */ + inc %ecx /* Existing GDT size */ + andl $~7, %ecx /* Ensure size is multiple of 8 */ + + subl %ecx, %esp /* Allocate new GDT */ + andl $~15, %esp /* Realign the stack */ + movl %esp, %edi /* New GDT address */ + leal 7(%ecx), %eax /* New GDT limit */ + pushw %cx /* Push 64-bit CS (for LJMP below) */ + pushl %edi /* Push new GDT address */ + pushw %ax /* Push new GDT limit */ + + /* Copy GDT to the stack and add a 64-bit code segment at the end */ + movl $GDT_ENTRY(DESC_CODE64, 0, 0xfffff) & 0xffffffff, (%edi,%ecx) + movl $GDT_ENTRY(DESC_CODE64, 0, 0xfffff) >> 32, 4(%edi,%ecx) + shrl $2, %ecx + cld + rep movsl /* Copy the firmware GDT */ + lgdtl (%esp) /* Switch to the new GDT */ call 1f 1: pop %edi + /* Record mixed mode entry */ + movb $0x0, (efi_is64 - 1b)(%edi) + + /* Set up indirect far call to re-enter 32-bit mode */ + leal (efi32_call - 1b)(%edi), %eax + addl %eax, (%eax) + movw %cs, 4(%eax) + /* Disable paging */ movl %cr0, %eax btrl $X86_CR0_PG_BIT, %eax @@ -297,8 +203,17 @@ SYM_FUNC_START_LOCAL(efi32_entry) movl %edx, (%eax) movl %eax, %cr3 - jmp startup_32 -SYM_FUNC_END(efi32_entry) + call efi32_enable_long_mode + + /* Set up far jump to 64-bit mode (CS is already on the stack) */ + leal (efi_stub_entry - 1b)(%edi), %eax + movl %eax, 2(%esp) + + movl 0(%ebp), %edi + movl 4(%ebp), %esi + movl %ebx, %edx + ljmpl *2(%esp) +SYM_FUNC_END(efi32_startup) /* * efi_status_t efi32_pe_entry(efi_handle_t image_handle, @@ -313,10 +228,8 @@ SYM_FUNC_START(efi32_pe_entry) btl $29, %edx // check long mode bit jnc 1f leal 8(%esp), %esp // preserve stack alignment - movl (%esp), %ecx // image_handle - movl 4(%esp), %edx // sys_table - jmp efi32_entry // pass %ecx, %edx - // no other registers remain live + xor %ebx, %ebx // no struct boot_params pointer + jmp efi32_startup // only ESP and EBX remain live 1: movl $0x80000003, %eax // EFI_UNSUPPORTED popl %ebx RET @@ -332,20 +245,10 @@ SYM_FUNC_END(efi64_stub_entry) .data .balign 8 -SYM_DATA_START_LOCAL(efi32_boot_gdt) - .word 0 - .quad 0 -SYM_DATA_END(efi32_boot_gdt) - -SYM_DATA_START_LOCAL(efi32_boot_idt) - .word 0 - .quad 0 -SYM_DATA_END(efi32_boot_idt) - -SYM_DATA_LOCAL(efi32_boot_cs, .word 0) -SYM_DATA_LOCAL(efi32_boot_ds, .word 0) -SYM_DATA_LOCAL(efi32_boot_sp, .long 0) -SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0, 0) +SYM_DATA_START_LOCAL(efi32_call) + .long efi_enter32 - . + .word 0x0 +SYM_DATA_END(efi32_call) SYM_DATA(efi_is64, .byte 1) .bss diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S index 1dcb794c5479..5db6495a3bb9 100644 --- a/arch/x86/boot/compressed/head_64.S +++ b/arch/x86/boot/compressed/head_64.S @@ -263,13 +263,6 @@ SYM_FUNC_START(startup_32) * used to perform that far jump. */ leal rva(startup_64)(%ebp), %eax -#ifdef CONFIG_EFI_MIXED - cmpb $1, rva(efi_is64)(%ebp) - je 1f - leal rva(startup_64_mixed_mode)(%ebp), %eax -1: -#endif - pushl $__KERNEL_CS pushl %eax -- 2.48.1.362.g079036d154-goog
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