UEFI使用ExitBootServices的简单示例(与gnu-efi一起)

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(我使用的是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: %rn", 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: %rn", status);
        return EFI_SUCCESS;
    }
    /* ExitBootServices */
    status = uefi_call_wrapper(SystemTable->BootServices->ExitBootServices,
        ImageHandle, MapKey);
    if (EFI_ERROR(status)) {
        Print(L"ExitBootServices: %rn", status);
        return EFI_SUCCESS;
    }
    write_string(0x07, "example");
}

甚至在执行ExitBootServices之前，qemu就崩溃了，错误如下:

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

有谁能告诉我我在做什么是错的吗?谢谢你。

Answer 1

看起来你的主要问题是你忘记在uefi_call_wrapper的参数的数量为你调用GetMemoryMap…传入一个指针(大数…)远远大于5)可能是垃圾UEFI固件仿真和QEMU的扩展。你的ExitBootServices调用也会因为同样的原因失败，因为你没有传入参数的数量。

你的代码还做了一些不必要的，可能不正确的假设…

1. 系统将在内存映射中有16个或更少的条目…
2. UEFI固件将返回你编译的EFI_MEMORY_DESCRIPTOR的任何版本…

GetMemoryMap的定义行为允许我们解决问题1，我们可以尽一切可能确保我们的代码与UEFI的新版本EFI_MEMORY_DESCRIPTOR的合理未来版本向前兼容。

下面是一个C语言中获取内存映射和退出引导服务的例子:

#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;
}