UEFI 使用 ExitBootServices 的简单示例（使用 gnu-efi）

Question

我正在尝试使用 gnu-efi 编写一个 hello world 类型的程序，但没有引导服务，因为它们在 ExitBootServices 之后变得不可用。在调用 ExitBootServices 之前直接写入显存不会显示任何内容。

出于这个原因，我需要调用 ExitBootServices，它需要一个 Mapkey。 MapKey 由 GetMemoryMap 函数提供。但是当我调用它时，我的应用程序崩溃了（我正在使用 qemu）。

这是我的代码：

#include <efi.h>
#include <efilib.h>

void write_string( int color, const char *string )
{
    volatile char *video = (volatile char*)0xB8000;
    while( *string != 0 )
    {
        *video++ = *string++;
        *video++ = color;
    }
}

EFI_STATUS
EFIAPI
efi_main (EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable)
{
    EFI_LOADED_IMAGE *loaded_image = NULL;
    EFI_STATUS status;
    InitializeLib(ImageHandle, SystemTable);

    status = uefi_call_wrapper(SystemTable->BootServices->HandleProtocol,
        3, ImageHandle, &LoadedImageProtocol, (void **)&loaded_image);
    if (EFI_ERROR(status)) {
        Print(L"handleprotocol: %r\n", status);
        return EFI_SUCCESS;
    }

    /* GetMemoryMap */
    UINTN MemoryMapSize = sizeof(EFI_MEMORY_DESCRIPTOR) * 0x10;
    EFI_MEMORY_DESCRIPTOR *MemoryMap = AllocatePool (MemoryMapSize);
    UINTN MapKey = 0;
    UINTN DescriptorSize = 0;
    UINT32 DescriptorVersion = 0;
    status = uefi_call_wrapper(SystemTable->BootServices->GetMemoryMap,
        &MemoryMapSize, MemoryMap, &MapKey, &DescriptorSize, &DescriptorVersion);
    if (EFI_ERROR(status)) {
        Print(L"GetMemoryMap: %r\n", status);
        return EFI_SUCCESS;
    }

    /* ExitBootServices */
    status = uefi_call_wrapper(SystemTable->BootServices->ExitBootServices,
        ImageHandle, MapKey);
    if (EFI_ERROR(status)) {
        Print(L"ExitBootServices: %r\n", status);
        return EFI_SUCCESS;
    }

    write_string(0x07, "example");
}

即使在执行 ExitBootServices 之前，qemu 也会因错误而崩溃：

qemu: fatal: Trying to execute code outside RAM or ROM at 0x00000000000b0000

谁能告诉我做错了什么？ 谢谢。

Answer 1

看起来您的主要问题是您忘记将参数数量传递给 uefi_call_wrapper 以调用 GetMemoryMap...传入指针（大数字...远大于 5）可能会破坏 UEFI固件仿真和 QEMU 扩展。由于同样的原因，您的 ExitBootServices 调用将失败，因为您没有传递参数的数量。

您的代码还做出了一些不必要的、可能不正确的假设...

1. 系统在内存映射中将有 16 个或更少的条目...
2. UEFI 固件将返回您编译时针对的任何版本的 EFI_MEMORY_DESCRIPTOR...

GetMemoryMap 的定义行为允许我们解决问题 1，并且我们可以尽一切可能确保我们的代码与未来合理的 EFI_MEMORY_DESCRIPTOR 版本向前兼容。

这是一个用 C 语言获取内存映射并退出引导服务的示例：

#include <efi.h>

#define ErrorCheck(actual, expected) if(actual != expected) return actual

EFI_STATUS EFIAPI efi_main(EFI_HANDLE imageHandle, EFI_SYSTEM_TABLE *systemTable)
{
    EFI_STATUS result;


   // TODO: Load anything that would change the memory map... (ex: OS kernal executable)


    UINTN mapSize = 0, mapKey, descriptorSize;
    EFI_MEMORY_DESCRIPTOR *memoryMap = NULL;
    UINT32 descriptorVersion;
    // Get the required memory pool size for the memory map...
    result = uefi_call_wrapper((void *)systemTable->BootServices->GetMemoryMap, 5, &mapSize, &memoryMap, NULL, &descriptorSize, NULL);
    ErrorCheck(result, EFI_BUFFER_TOO_SMALL);
    // Allocating the pool creates at least one new descriptor... for the chunk of memory changed to EfiLoaderData
    // Not sure that UEFI firmware must allocate on a memory type boundry... if not, then two descriptors might be created
    mapSize += 2 * descriptorSize;
    // Get a pool of memory to hold the map...
    result = uefi_call_wrapper((void *)systemTable->BootServices->AllocatePool, 3, EfiLoaderData, mapSize, (void **)&memoryMap);
    ErrorCheck(result, EFI_SUCCESS);
    // Get the actual memory map...
    result = uefi_call_wrapper((void *)systemTable->BootServices->GetMemoryMap, 5, &mapSize, &memoryMap, &mapKey, &descriptorSize, &descriptorVersion);
    ErrorCheck(result, EFI_SUCCESS);

    result = uefi_call_wrapper((void *)systemTable->BootServices->ExitBootServices, 2, imageHandle, mapKey);
    ErrorCheck(result, EFI_SUCCESS);


    // TODO: Boot Services no longer available. Do whatever with Runtime Services... (ex: start OS kernal executable)


    return EFI_SUCCESS;
}