Linux 如何使用O_DIRECT将内核空间内存(物理地址)写入文件?
我想将物理内存写入文件。内存本身不会再被触摸,因此我想使用Linux 如何使用O_DIRECT将内核空间内存(物理地址)写入文件?,linux,kernel,kernel-module,dma,Linux,Kernel,Kernel Module,Dma,我想将物理内存写入文件。内存本身不会再被触摸,因此我想使用O_DIRECT来获得最佳的写入性能 我的第一个想法是打开/dev/mem并对内存进行mmap,然后将所有内容写入一个文件,该文件用O_DIRECT打开。对mmap返回的内存地址的写入调用失败(EFAULT)。如果我不使用O_DIRECT,它会导致memcpy #include <cstdint> #include <iostream> #include <unistd.h> #include <
O_DIRECT
来获得最佳的写入性能
我的第一个想法是打开/dev/mem
并对内存进行mmap,然后将所有内容写入一个文件,该文件用O_DIRECT
打开。对mmap返回的内存地址的写入调用失败(EFAULT
)。如果我不使用O_DIRECT
,它会导致memcpy
#include <cstdint>
#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>
#include <malloc.h>
#include <sys/mman.h>
#define PRINT_ERRNO_REASON(reason) \
case reason: { std::cout << #reason << std::endl; } break;
void write_page_aligned_buffer(int out_fd)
{
const ssize_t PAGE_SIZE = getpagesize();
void* page_aligned_buffer = memalign(PAGE_SIZE, PAGE_SIZE);
if(!page_aligned_buffer)
{
std::cout << "Could not allocate page aligned buffer." << std::endl;
return;
}
std::cout << "Allocated a buffer at address " << page_aligned_buffer << "." << std::endl;
if(write(out_fd, page_aligned_buffer, PAGE_SIZE) < 0)
{
std::cout << "Could not write page-aligned user buffer to tmp-file. Quitting..." << std::endl;
std::cout << "Reason of fail is ";
switch(errno)
{
PRINT_ERRNO_REASON(EAGAIN);
PRINT_ERRNO_REASON(EBADF);
PRINT_ERRNO_REASON(EFAULT);
PRINT_ERRNO_REASON(EFBIG);
PRINT_ERRNO_REASON(EINTR);
PRINT_ERRNO_REASON(EINVAL);
PRINT_ERRNO_REASON(EIO);
PRINT_ERRNO_REASON(ENOSPC);
PRINT_ERRNO_REASON(EPIPE);
default:
std::cout << "Unknown" << std::endl;
}
}
else
{
std::cout << "Successfully written user-page-aligned buffer." << std::endl;
}
free(page_aligned_buffer);
}
int main()
{
const ssize_t PAGE_SIZE = getpagesize();
// number of pages to copy
const uint32_t PAGES_TO_COPY = 1;
char* tmp_file_name = 0;
int tmp_file_fd = -1;
ssize_t bytes_copied = 0;
std::cout << "Copying " << PAGES_TO_COPY << " pages with PAGE_SIZE = " << PAGE_SIZE << std::endl;
std::cout << "Copying " << PAGES_TO_COPY * PAGE_SIZE / 1024 << " kBytes." << std::endl << std::endl;
uid_t user_id = geteuid();
if(user_id)
{
std::cout << "We need to be root. I am euid == " << user_id << ". Quitting..." << std::endl;
return 0;
}
else
{
seteuid(0);
setuid(0);
}
// get the file descriptor
int mem_fd = open("/dev/mem", O_RDONLY);
if(mem_fd < 0)
{
std::cout << "Could not open /dev/mem. Quitting..." << std::endl;
std::cout << "Reason of fail is ";
switch(errno)
{
PRINT_ERRNO_REASON(EACCES);
PRINT_ERRNO_REASON(EEXIST);
PRINT_ERRNO_REASON(EINTR);
PRINT_ERRNO_REASON(EINVAL);
PRINT_ERRNO_REASON(EIO);
PRINT_ERRNO_REASON(EISDIR);
PRINT_ERRNO_REASON(ELOOP);
PRINT_ERRNO_REASON(EMFILE);
PRINT_ERRNO_REASON(ENFILE);
PRINT_ERRNO_REASON(ENOENT);
PRINT_ERRNO_REASON(ENOSR);
PRINT_ERRNO_REASON(ENOSPC);
PRINT_ERRNO_REASON(ENOTDIR);
PRINT_ERRNO_REASON(ENXIO);
PRINT_ERRNO_REASON(EOVERFLOW);
PRINT_ERRNO_REASON(EROFS);
PRINT_ERRNO_REASON(EAGAIN);
PRINT_ERRNO_REASON(ENAMETOOLONG);
PRINT_ERRNO_REASON(ENOMEM);
PRINT_ERRNO_REASON(ETXTBSY);
default:
std::cout << "Unknown" << std::endl;
}
return 0;
}
// get read pointer
uint8_t* mem_ptr = static_cast<uint8_t*>(mmap(0,
PAGE_SIZE,
PROT_READ,
MAP_SHARED,
mem_fd,
PAGE_SIZE));
if(mem_ptr == MAP_FAILED)
{
std::cout << "Could not mmap /dev/mem. Quitting..." << std::endl;
std::cout << "Reason of fail is ";
switch(errno)
{
PRINT_ERRNO_REASON(EACCES);
PRINT_ERRNO_REASON(EAGAIN);
PRINT_ERRNO_REASON(EBADF);
PRINT_ERRNO_REASON(EINVAL);
PRINT_ERRNO_REASON(ENFILE);
PRINT_ERRNO_REASON(ENODEV);
PRINT_ERRNO_REASON(ENOMEM);
PRINT_ERRNO_REASON(EPERM);
PRINT_ERRNO_REASON(ETXTBSY);
default:
std::cout << "Unknown" << std::endl;
}
goto CLEANUP_FD_DEV_MEM;
}
tmp_file_name = tempnam("/tmp", "prefix");
if(!tmp_file_name)
{
std::cout << "Could not get a free tmp-filename";
goto CLEANUP_MMAP_DEV_MEM;
}
// if O_DIRECT is omitted the example will work
tmp_file_fd = open(tmp_file_name,
O_WRONLY | O_CREAT | O_DIRECT | O_TRUNC,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
if(tmp_file_fd < 0)
{
std::cout << "Could not create tmp file with O_DIRECT." << std::endl;
goto CLEANUP_MMAP_DEV_MEM;
}
write_page_aligned_buffer(tmp_file_fd);
// everything worked so lets start the copying
for(uint i = 0; i < PAGES_TO_COPY; i++)
{
// check memory
// snip
for(int i = 0; i < PAGE_SIZE; i += 32)
{
printf("%02X%02X%02X%02X %02X%02X%02X%02X %02X%02X%02X%02X %02X%02X%02X%02X %02X%02X%02X%02X %02X%02X%02X%02X %02X%02X%02X%02X %02X%02X%02X%02X\n",
mem_ptr[i + 0], mem_ptr[i + 1], mem_ptr[i + 2], mem_ptr[i + 3],
mem_ptr[i + 4], mem_ptr[i + 5], mem_ptr[i + 6], mem_ptr[i + 7],
mem_ptr[i + 8], mem_ptr[i + 9], mem_ptr[i + 10], mem_ptr[i + 11],
mem_ptr[i + 12], mem_ptr[i + 13], mem_ptr[i + 14], mem_ptr[i + 15],
mem_ptr[i + 16], mem_ptr[i + 17], mem_ptr[i + 18], mem_ptr[i + 19],
mem_ptr[i + 20], mem_ptr[i + 21], mem_ptr[i + 22], mem_ptr[i + 23],
mem_ptr[i + 24], mem_ptr[i + 25], mem_ptr[i + 26], mem_ptr[i + 27],
mem_ptr[i + 28], mem_ptr[i + 29], mem_ptr[i + 30], mem_ptr[i + 31]);
}
std::cout << "\n";
// endsnip
bytes_copied = write(tmp_file_fd, mem_ptr, PAGE_SIZE);
if(bytes_copied < 0)
{
std::cout << "Could not write to tmp-file. Quitting..." << std::endl;
std::cout << "Reason of fail is ";
switch(errno)
{
PRINT_ERRNO_REASON(EAGAIN);
PRINT_ERRNO_REASON(EBADF);
PRINT_ERRNO_REASON(EFAULT);
PRINT_ERRNO_REASON(EFBIG);
PRINT_ERRNO_REASON(EINTR);
PRINT_ERRNO_REASON(EINVAL);
PRINT_ERRNO_REASON(EIO);
PRINT_ERRNO_REASON(ENOSPC);
PRINT_ERRNO_REASON(EPIPE);
default:
std::cout << "Unknown" << std::endl;
}
goto CLEANUP_FD_TMP_FILE;
}
}
CLEANUP_FD_TMP_FILE:
if(tmp_file_name)
{
if(close(tmp_file_fd))
{
std::cout << "Could close tmp-file " << tmp_file_name << "." << std::endl;
}
if(remove(tmp_file_name))
{
std::cout << "Could remove tmp-file " << tmp_file_name << "." << std::endl;
}
free(tmp_file_name);
}
CLEANUP_MMAP_DEV_MEM:
if(munmap(mem_ptr, PAGE_SIZE))
{
std::cout << "munmap failed." << std::endl;
}
CLEANUP_FD_DEV_MEM:
if(close(mem_fd))
{
std::cout << "Could not close /dev/mem filedescriptor." << std::endl;
}
return 0;
}
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#定义打印错误原因(原因)\
案例理由:{STD::Couth我今晚测试了,如果使用OXONDION,你的写作将是有条件的。
< P>你的问题看起来有点奇怪。因为你的编程相当接近硬件,你可以考虑使用直接内存访问(DMA)。。这可能有点棘手,因为您需要了解分页和部分I/O机制。您可能需要阅读:
(这只是一个简单的介绍。)写入硬盘比memcpy快?你是认真的吗?是的,它是基于FPGA的设计(不是快速CPU),带有嵌入式SATA控制器和真正快速的SSD。@Friedrich你能写()另一个缓冲区(对齐良好)您的文件是用O_DIRECT打开的吗?是的,请参阅我编辑的文章。可能splice/VMPlice/tee是一个选项,例如,一个实现splice_读取功能的字符驱动程序。Linux文件系统已经支持splice。另外,请看一下busybox devmem
source正在做什么: