source: benchmark/io/io_uring.h@ f03209d3

extern "C" {
        #ifndef _GNU_SOURCE         /* See feature_test_macros(7) */
        #define _GNU_SOURCE         /* See feature_test_macros(7) */
        #endif
        #include <errno.h>
        #include <stdio.h>
        #include <stdint.h>
        #include <stdlib.h>
        #include <string.h>
        #include <unistd.h>
        #include <sys/mman.h>
        #include <sys/syscall.h>
        #include <sys/uio.h>
        #include <fcntl.h>

        #include <linux/io_uring.h>
}

# ifndef __NR_io_uring_setup
#  define __NR_io_uring_setup           425
# endif
# ifndef __NR_io_uring_enter
#  define __NR_io_uring_enter           426
# endif
# ifndef __NR_io_uring_register
#  define __NR_io_uring_register        427
# endif

struct io_uring_sq {
        // Head and tail of the ring (associated with array)
        volatile uint32_t * head;
        volatile uint32_t * tail;

        // The actual kernel ring which uses head/tail
        // indexes into the sqes arrays
        uint32_t * array;

        // number of entries and mask to go with it
        const uint32_t * num;
        const uint32_t * mask;

        // Submission flags (Not sure what for)
        uint32_t * flags;

        // number of sqes not submitted (whatever that means)
        uint32_t * dropped;

        // Like head/tail but not seen by the kernel
        volatile uint32_t alloc;

        // A buffer of sqes (not the actual ring)
        struct io_uring_sqe * sqes;

        // The location and size of the mmaped area
        void * ring_ptr;
        size_t ring_sz;
};

struct io_uring_cq {
        // Head and tail of the ring
        volatile uint32_t * head;
        volatile uint32_t * tail;

        // number of entries and mask to go with it
        const uint32_t * mask;
        const uint32_t * num;

        // number of cqes not submitted (whatever that means)
        uint32_t * overflow;

        // the kernel ring
        struct io_uring_cqe * cqes;

        // The location and size of the mmaped area
        void * ring_ptr;
        size_t ring_sz;
};

struct io_ring {
        struct io_uring_sq submit_q;
        struct io_uring_cq completion_q;
        uint32_t flags;
        int fd;
};

struct IO_singleton {
        io_ring io;
};

IO_singleton self;

void init_uring(uint32_t nentries) {
      // Step 1 : call to setup
        struct io_uring_params params;
        memset(&params, 0, sizeof(params));
        // params.flags = IORING_SETUP_SQPOLL;

        int fd = syscall(__NR_io_uring_setup, nentries, &params );
        if(fd < 0) {
                fprintf(stderr, "KERNEL ERROR: IO_URING SETUP - %s\n", strerror(errno));
                abort();
        }

        // Step 2 : mmap result
        memset(&self.io, 0, sizeof(struct io_ring));
        struct io_uring_sq & sq = self.io.submit_q;
        struct io_uring_cq & cq = self.io.completion_q;

        // calculate the right ring size
        sq.ring_sz = params.sq_off.array + (params.sq_entries * sizeof(unsigned)           );
        cq.ring_sz = params.cq_off.cqes  + (params.cq_entries * sizeof(struct io_uring_cqe));

        // Requires features
        // // adjust the size according to the parameters
        // if ((params.features & IORING_FEAT_SINGLE_MMAP) != 0) {
        //      cq->ring_sz = sq->ring_sz = max(cq->ring_sz, sq->ring_sz);
        // }

        // mmap the Submit Queue into existence
        sq.ring_ptr = mmap(0, sq.ring_sz, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE, fd, IORING_OFF_SQ_RING);
        if (sq.ring_ptr == (void*)MAP_FAILED) {
                fprintf(stderr, "KERNEL ERROR: IO_URING MMAP1 - %s\n", strerror(errno));
                abort();
        }

        // mmap the Completion Queue into existence (may or may not be needed)
        // Requires features
        // if ((params.features & IORING_FEAT_SINGLE_MMAP) != 0) {
        //      cq->ring_ptr = sq->ring_ptr;
        // }
        // else {
                // We need multiple call to MMAP
                cq.ring_ptr = mmap(0, cq.ring_sz, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE, fd, IORING_OFF_CQ_RING);
                if (cq.ring_ptr == (void*)MAP_FAILED) {
                        munmap(sq.ring_ptr, sq.ring_sz);
                        fprintf(stderr, "KERNEL ERROR: IO_URING MMAP2 - %s\n", strerror(errno));
                        abort();
                }
        // }

        // mmap the submit queue entries
        size_t size = params.sq_entries * sizeof(struct io_uring_sqe);
        sq.sqes = (struct io_uring_sqe *)mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE, fd, IORING_OFF_SQES);
        if (sq.sqes == (struct io_uring_sqe *)MAP_FAILED) {
                munmap(sq.ring_ptr, sq.ring_sz);
                if (cq.ring_ptr != sq.ring_ptr) munmap(cq.ring_ptr, cq.ring_sz);
                fprintf(stderr, "KERNEL ERROR: IO_URING MMAP3 - %s\n", strerror(errno));
                abort();
        }

        // Get the pointers from the kernel to fill the structure
        // submit queue
        sq.head    = (volatile uint32_t *)(((intptr_t)sq.ring_ptr) + params.sq_off.head);
        sq.tail    = (volatile uint32_t *)(((intptr_t)sq.ring_ptr) + params.sq_off.tail);
        sq.mask    = (   const uint32_t *)(((intptr_t)sq.ring_ptr) + params.sq_off.ring_mask);
        sq.num     = (   const uint32_t *)(((intptr_t)sq.ring_ptr) + params.sq_off.ring_entries);
        sq.flags   = (         uint32_t *)(((intptr_t)sq.ring_ptr) + params.sq_off.flags);
        sq.dropped = (         uint32_t *)(((intptr_t)sq.ring_ptr) + params.sq_off.dropped);
        sq.array   = (         uint32_t *)(((intptr_t)sq.ring_ptr) + params.sq_off.array);
        sq.alloc = *sq.tail;

        // completion queue
        cq.head     = (volatile uint32_t *)(((intptr_t)cq.ring_ptr) + params.cq_off.head);
        cq.tail     = (volatile uint32_t *)(((intptr_t)cq.ring_ptr) + params.cq_off.tail);
        cq.mask     = (   const uint32_t *)(((intptr_t)cq.ring_ptr) + params.cq_off.ring_mask);
        cq.num      = (   const uint32_t *)(((intptr_t)cq.ring_ptr) + params.cq_off.ring_entries);
        cq.overflow = (         uint32_t *)(((intptr_t)cq.ring_ptr) + params.cq_off.overflow);
        cq.cqes   = (struct io_uring_cqe *)(((intptr_t)cq.ring_ptr) + params.cq_off.cqes);

        self.io.fd = fd;
}