在x86平台上使用-mcmodel = kernel标志 [英] use of -mcmodel=kernel flag in x86 platform
问题描述
我试图交叉编译为x86架构构建的设备驱动程序，以武装平台.它已编译，没有任何错误，但我认为所有功能均不可用.因此，我检查了makefile并找到了该特定部分.
I am trying to cross compile a device driver built for x86 architecture to arm platform. It got compiled without any errors, but I dont think whole features are available. So I checked the makefile and found this particular part.
ifeq ($(ARCH),x86_64)
EXTRA_CFLAGS += -mcmodel=kernel -mno-red-zone
这是唯一看起来依赖于体系结构的部分.在Google上待了一段时间后，我发现-mcmodel = kernel用于内核代码模型，而-mno-red-zone是为了避免在内存中使用红色区域，并且它们都用于x86_64.但是我不清楚，将cmodel设置为内核会产生什么影响?
This is the only part that depends on architecture it seems. After some time on google, I found that -mcmodel=kernel is for kernel code model and -mno-red-zone is to avoid using red zone in memory and both them were for x86_64. But its not clear to me, what impact does it make setting cmodel to kernel?
(对手臂问题的任何见解也将不胜感激.)
(Any insight into the problem with arm is also greatly appreciated.)
推荐答案
-mcmodel=kernel
为内核代码模型生成代码. 内核在负2 GB的地址空间中运行.
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space.
(即高2GiB，类似0xfffffffff0001234的地址)
(i.e. the upper 2GiB, addresses like 0xfffffffff0001234)
在内核代码模型中，静态符号地址不适合32位零扩展常量(与默认的小代码模型不同，在默认小代码模型中，mov eax, imm32(5个字节)是在其中放置符号地址的最有效方法)一个寄存器).
In the kernel code model, static symbol addresses don't fit in 32-bit zero-extended constants (unlike the default small code model where mov eax, imm32 (5 bytes) is the most efficient way to put a symbol address in a register).
但是它们 do 符合符号扩展的32位常量，例如，不同于large代码模型.因此，mov rax, sign_extended_imm32(7个字节)有效，并且大小相同，但可能比lea rax, [rel symbol]效率更高.
But they do fit in sign-extended 32-bit constants, unlike the large code model for example. So mov rax, sign_extended_imm32 (7 bytes) works, and is the same size but maybe slightly more efficient than lea rax, [rel symbol].
但更重要的是mov eax, [table + rdi*4]可以工作，因为disp32位移被符号扩展为64位. -mcmodel=kernel告诉gcc可以执行此操作，但mov eax, table 不能.
But more importantly mov eax, [table + rdi*4] works, because disp32 displacements are sign-extended to 64 bits. -mcmodel=kernel tells gcc it can do this but not mov eax, table.
相对于RIP的寻址也可以到达任何代码地址中的任何符号(具有rel32 + -2GiB偏移)，因此-fPIC或-fPIE也会使您的代码正常工作，而付出的代价很小，就是不利用以下代码:在有用的情况下使用32位绝对寻址. (例如，索引静态数组).
RIP-relative addressing can also reach any symbol from any code address (with a rel32 +-2GiB offset), so -fPIC or -fPIE will also make your code work, at the minor expense of not taking advantage of 32-bit absolute addressing in cases where it's useful. (e.g. indexing static arrays).
如果没有-mcmodel=kernel(像这样)，您不会收到链接错误，您可能有一个通过PIE可执行文件的gcc默认值(在最近的发行版中很常见)，因此避免了绝对寻址.
If you didn't get link errors without -mcmodel=kernel (like these), you probably have a gcc that makes PIE executables by default (common on recent distros), so it avoids absolute addressing.
这篇关于在x86平台上使用-mcmodel = kernel标志的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
