cat_io_uring.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <linux/fs.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

/* If your compilation fails because the header file below is missing,
 * your kernel is probably too old to support io_uring.
 * */
#include <linux/io_uring.h>

#define QUEUE_DEPTH 1
#define BLOCK_SZ    1024

/* This is x86 specific */
#define read_barrier()  __asm__ __volatile__("":::"memory")
#define write_barrier() __asm__ __volatile__("":::"memory")

struct app_io_sq_ring {
    unsigned *head;
    unsigned *tail;
    unsigned *ring_mask;
    unsigned *ring_entries;
    unsigned *flags;
    unsigned *array;
};

struct app_io_cq_ring {
    unsigned *head;
    unsigned *tail;
    unsigned *ring_mask;
    unsigned *ring_entries;
    struct io_uring_cqe *cqes;
};

struct submitter {
    int ring_fd;
    struct app_io_sq_ring sq_ring;
    struct io_uring_sqe *sqes;
    struct app_io_cq_ring cq_ring;
};

struct file_info {
    off_t file_sz;
    struct iovec iovecs[];      /* Referred by readv/writev */
};

/*
 * This code is written in the days when io_uring-related system calls are not
 * part of standard C libraries. So, we roll our own system call wrapper
 * functions.
 * */

int io_uring_setup(unsigned entries, struct io_uring_params *p)
{
    return (int) syscall(__NR_io_uring_setup, entries, p);
}

int io_uring_enter(int ring_fd, unsigned int to_submit,
                          unsigned int min_complete, unsigned int flags)
{
    return (int) syscall(__NR_io_uring_enter, ring_fd, to_submit, min_complete,
                   flags, NULL, 0);
}

/*
 * Returns the size of the file whose open file descriptor is passed in.
 * Properly handles regular file and block devices as well. Pretty.
 * */

off_t get_file_size(int fd) {
    struct stat st;

    if(fstat(fd, &st) < 0) {
        perror("fstat");
        return -1;
    }
    if (S_ISBLK(st.st_mode)) {
        unsigned long long bytes;
        if (ioctl(fd, BLKGETSIZE64, &bytes) != 0) {
            perror("ioctl");
            return -1;
        }
        return bytes;
    } else if (S_ISREG(st.st_mode))
        return st.st_size;

    return -1;
}

/*
 * io_uring requires a lot of setup which looks pretty hairy, but isn't all
 * that difficult to understand. Because of all this boilerplate code,
 * io_uring's author has created liburing, which is relatively easy to use.
 * However, you should take your time and understand this code. It is always
 * good to know how it all works underneath. Apart from bragging rights,
 * it does offer you a certain strange geeky peace.
 * */

int app_setup_uring(struct submitter *s) {
    struct app_io_sq_ring *sring = &s->sq_ring;
    struct app_io_cq_ring *cring = &s->cq_ring;
    struct io_uring_params p;
    void *sq_ptr, *cq_ptr;

    /*
     * We need to pass in the io_uring_params structure to the io_uring_setup()
     * call zeroed out. We could set any flags if we need to, but for this
     * example, we don't.
     * */
    memset(&p, 0, sizeof(p));
    s->ring_fd = io_uring_setup(QUEUE_DEPTH, &p);
    if (s->ring_fd < 0) {
        perror("io_uring_setup");
        return 1;
    }

    /*
     * io_uring communication happens via 2 shared kernel-user space ring buffers,
     * which can be jointly mapped with a single mmap() call in recent kernels. 
     * While the completion queue is directly manipulated, the submission queue 
     * has an indirection array in between. We map that in as well.
     * */

    int sring_sz = p.sq_off.array + p.sq_entries * sizeof(unsigned);
    int cring_sz = p.cq_off.cqes + p.cq_entries * sizeof(struct io_uring_cqe);

    /* In kernel version 5.4 and above, it is possible to map the submission and 
     * completion buffers with a single mmap() call. Rather than check for kernel 
     * versions, the recommended way is to just check the features field of the 
     * io_uring_params structure, which is a bit mask. If the 
     * IORING_FEAT_SINGLE_MMAP is set, then we can do away with the second mmap()
     * call to map the completion ring.
     * */
    if (p.features & IORING_FEAT_SINGLE_MMAP) {
        if (cring_sz > sring_sz) {
            sring_sz = cring_sz;
        }
        cring_sz = sring_sz;
    }

    /* Map in the submission and completion queue ring buffers.
     * Older kernels only map in the submission queue, though.
     * */
    sq_ptr = mmap(0, sring_sz, PROT_READ | PROT_WRITE, 
            MAP_SHARED | MAP_POPULATE,
            s->ring_fd, IORING_OFF_SQ_RING);
    if (sq_ptr == MAP_FAILED) {
        perror("mmap");
        return 1;
    }

    if (p.features & IORING_FEAT_SINGLE_MMAP) {
        cq_ptr = sq_ptr;
    } else {
        /* Map in the completion queue ring buffer in older kernels separately */
        cq_ptr = mmap(0, cring_sz, PROT_READ | PROT_WRITE, 
                MAP_SHARED | MAP_POPULATE,
                s->ring_fd, IORING_OFF_CQ_RING);
        if (cq_ptr == MAP_FAILED) {
            perror("mmap");
            return 1;
        }
    }
    /* Save useful fields in a global app_io_sq_ring struct for later
     * easy reference */
    sring->head = sq_ptr + p.sq_off.head;
    sring->tail = sq_ptr + p.sq_off.tail;
    sring->ring_mask = sq_ptr + p.sq_off.ring_mask;
    sring->ring_entries = sq_ptr + p.sq_off.ring_entries;
    sring->flags = sq_ptr + p.sq_off.flags;
    sring->array = sq_ptr + p.sq_off.array;

    /* Map in the submission queue entries array */
    s->sqes = mmap(0, p.sq_entries * sizeof(struct io_uring_sqe),
            PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE,
            s->ring_fd, IORING_OFF_SQES);
    if (s->sqes == MAP_FAILED) {
        perror("mmap");
        return 1;
    }

    /* Save useful fields in a global app_io_cq_ring struct for later
     * easy reference */
    cring->head = cq_ptr + p.cq_off.head;
    cring->tail = cq_ptr + p.cq_off.tail;
    cring->ring_mask = cq_ptr + p.cq_off.ring_mask;
    cring->ring_entries = cq_ptr + p.cq_off.ring_entries;
    cring->cqes = cq_ptr + p.cq_off.cqes;

    return 0;
}

/*
 * Output a string of characters of len length to stdout.
 * We use buffered output here to be efficient,
 * since we need to output character-by-character.
 * */
void output_to_console(char *buf, int len) {
    while (len--) {
        fputc(*buf++, stdout);
    }
}

/*
 * Read from completion queue.
 * In this function, we read completion events from the completion queue, get
 * the data buffer that will have the file data and print it to the console.
 * */

void read_from_cq(struct submitter *s) {
    struct file_info *fi;
    struct app_io_cq_ring *cring = &s->cq_ring;
    struct io_uring_cqe *cqe;
    unsigned head, reaped = 0;

    head = *cring->head;

    do {
        read_barrier();
        /*
         * Remember, this is a ring buffer. If head == tail, it means that the
         * buffer is empty.
         * */
        if (head == *cring->tail)
            break;

        /* Get the entry */
        cqe = &cring->cqes[head & *s->cq_ring.ring_mask];
        fi = (struct file_info*) cqe->user_data;
        if (cqe->res < 0)
            fprintf(stderr, "Error: %s\n", strerror(abs(cqe->res)));

        int blocks = (int) fi->file_sz / BLOCK_SZ;
        if (fi->file_sz % BLOCK_SZ) blocks++;

        for (int i = 0; i < blocks; i++)
            output_to_console(fi->iovecs[i].iov_base, fi->iovecs[i].iov_len);

        head++;
    } while (1);

    *cring->head = head;
    write_barrier();
}
/*
 * Submit to submission queue.
 * In this function, we submit requests to the submission queue. You can submit
 * many types of requests. Ours is going to be the readv() request, which we
 * specify via IORING_OP_READV.
 *
 * */
int submit_to_sq(char *file_path, struct submitter *s) {
    struct file_info *fi;

    int file_fd = open(file_path, O_RDONLY);
    if (file_fd < 0 ) {
        perror("open");
        return 1;
    }

    struct app_io_sq_ring *sring = &s->sq_ring;
    unsigned index = 0, current_block = 0, tail = 0, next_tail = 0;

    off_t file_sz = get_file_size(file_fd);
    if (file_sz < 0)
        return 1;
    off_t bytes_remaining = file_sz;
    int blocks = (int) file_sz / BLOCK_SZ;
    if (file_sz % BLOCK_SZ) blocks++;

    fi = malloc(sizeof(*fi) + sizeof(struct iovec) * blocks);
    if (!fi) {
        fprintf(stderr, "Unable to allocate memory\n");
        return 1;
    }
    fi->file_sz = file_sz;

    /*
     * For each block of the file we need to read, we allocate an iovec struct
     * which is indexed into the iovecs array. This array is passed in as part
     * of the submission. If you don't understand this, then you need to look
     * up how the readv() and writev() system calls work.
     * */
    while (bytes_remaining) {
        off_t bytes_to_read = bytes_remaining;
        if (bytes_to_read > BLOCK_SZ)
            bytes_to_read = BLOCK_SZ;

        fi->iovecs[current_block].iov_len = bytes_to_read;

        void *buf;
        if( posix_memalign(&buf, BLOCK_SZ, BLOCK_SZ)) {
            perror("posix_memalign");
            return 1;
        }
        fi->iovecs[current_block].iov_base = buf;

        current_block++;
        bytes_remaining -= bytes_to_read;
    }

    /* Add our submission queue entry to the tail of the SQE ring buffer */
    next_tail = tail = *sring->tail;
    next_tail++;
    read_barrier();
    index = tail & *s->sq_ring.ring_mask;
    struct io_uring_sqe *sqe = &s->sqes[index];
    sqe->fd = file_fd;
    sqe->flags = 0;
    sqe->opcode = IORING_OP_READV;
    sqe->addr = (unsigned long) fi->iovecs;
    sqe->len = blocks;
    sqe->off = 0;
    sqe->user_data = (unsigned long long) fi;
    sring->array[index] = index;
    tail = next_tail;

    /* Update the tail so the kernel can see it. */
    if(*sring->tail != tail) {
        *sring->tail = tail;
        write_barrier();
    }

    /*
     * Tell the kernel we have submitted events with the io_uring_enter() system
     * call. We also pass in the IOURING_ENTER_GETEVENTS flag which causes the
     * io_uring_enter() call to wait until min_complete events (the 3rd param)
     * complete.
     * */
    int ret =  io_uring_enter(s->ring_fd, 1,1,
            IORING_ENTER_GETEVENTS);
    if(ret < 0) {
        perror("io_uring_enter");
        return 1;
    }

    return 0;
}

int main(int argc, char *argv[]) {
    struct submitter s;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <filename>\n", argv[0]);
        return 1;
    }

    memset(&s, 0, sizeof(s));

    if(app_setup_uring(&s)) {
        fprintf(stderr, "Unable to setup uring!\n");
        return 1;
    }

    for (int i = 1; i < argc; i++) {
        if(submit_to_sq(argv[i], &s)) {
            fprintf(stderr, "Error reading file\n");
            return 1;
        }
        read_from_cq(&s);
    }

    return 0;
}