xen/extras/mini-os/gnttab.c

/* 
 ****************************************************************************
 * (C) 2006 - Cambridge University
 ****************************************************************************
 *
 *        File: gnttab.c
 *      Author: Steven Smith (sos22@cam.ac.uk) 
 *     Changes: Grzegorz Milos (gm281@cam.ac.uk)
 *              
 *        Date: July 2006
 * 
 * Environment: Xen Minimal OS
 * Description: Simple grant tables implementation. About as stupid as it's
 *  possible to be and still work.
 *
 ****************************************************************************
 */
#include <os.h>
#include <mm.h>
#include <gnttab.h>

#define NR_RESERVED_ENTRIES 8

#define NR_GRANT_FRAMES 4
#define NR_GRANT_ENTRIES (NR_GRANT_FRAMES * PAGE_SIZE / sizeof(grant_entry_t))
#define GNTTAB_LIST_END (NR_GRANT_ENTRIES + 1)

static grant_entry_t *gnttab_table;
static grant_ref_t gnttab_list[NR_GRANT_ENTRIES];
static grant_ref_t gnttab_free_head;

static grant_ref_t
get_free_entries(int count)
{
    grant_ref_t ref;
    grant_ref_t head;

    ref = head = gnttab_free_head;
    while (count-- > 1)
	head = gnttab_list[head];
    gnttab_free_head = gnttab_list[head];
    gnttab_list[head] = GNTTAB_LIST_END;
    return ref;
}

static void
put_free_entry(grant_ref_t gref)
{
    gnttab_list[gref] = gnttab_free_head;
    gnttab_free_head = gref;
}

grant_ref_t
gnttab_grant_access(domid_t domid, unsigned long frame, int readonly)
{
    grant_ref_t ref;

    ref = get_free_entries(1);
    gnttab_table[ref].frame = frame;
    gnttab_table[ref].domid = domid;
    wmb();
    readonly *= GTF_readonly;
    gnttab_table[ref].flags = GTF_permit_access | readonly;

    return ref;
}

grant_ref_t
gnttab_grant_transfer(domid_t domid, unsigned long pfn)
{
    grant_ref_t ref;

    ref = get_free_entries(1);
    gnttab_table[ref].frame = pfn;
    gnttab_table[ref].domid = domid;
    wmb();
    gnttab_table[ref].flags = GTF_accept_transfer;

    return ref;
}

int
gnttab_end_access(grant_ref_t ref)
{
    u16 flags, nflags;

    nflags = gnttab_table[ref].flags;
    do {
        if ((flags = nflags) & (GTF_reading|GTF_writing)) {
            printk("WARNING: g.e. still in use!\n");
            return 0;
        }
    } while ((nflags = synch_cmpxchg(&gnttab_table[ref].flags, flags, 0)) !=
            flags);

    put_free_entry(ref);
    return 1;
}

unsigned long
gnttab_end_transfer(grant_ref_t ref)
{
    unsigned long frame;
    u16 flags;

    while (!((flags = gnttab_table[ref].flags) & GTF_transfer_committed)) {
        if (synch_cmpxchg(&gnttab_table[ref].flags, flags, 0) == flags) {
            printk("Release unused transfer grant.\n");
            put_free_entry(ref);
            return 0;
        }
    }

    /* If a transfer is in progress then wait until it is completed. */
    while (!(flags & GTF_transfer_completed)) {
        flags = gnttab_table[ref].flags;
    }

    /* Read the frame number /after/ reading completion status. */
    rmb();
    frame = gnttab_table[ref].frame;

    put_free_entry(ref);

    return frame;
}

grant_ref_t
gnttab_alloc_and_grant(void **map)
{
    unsigned long mfn;
    grant_ref_t gref;

    *map = (void *)alloc_page();
    mfn = virt_to_mfn(*map);
    gref = gnttab_grant_access(0, mfn, 0);
    return gref;
}

static const char *gnttabop_error_msgs[] = GNTTABOP_error_msgs;

const char *
gnttabop_error(int16_t status)
{
    status = -status;
    if (status < 0 || status >= ARRAY_SIZE(gnttabop_error_msgs))
	return "bad status";
    else
        return gnttabop_error_msgs[status];
}

void
init_gnttab(void)
{
    struct gnttab_setup_table setup;
    unsigned long frames[NR_GRANT_FRAMES];
    int i;

    for (i = NR_RESERVED_ENTRIES; i < NR_GRANT_ENTRIES; i++)
	gnttab_list[i] = i + 1;
    gnttab_free_head = NR_RESERVED_ENTRIES;

    setup.dom = DOMID_SELF;
    setup.nr_frames = NR_GRANT_FRAMES;
    set_xen_guest_handle(setup.frame_list, frames);

    HYPERVISOR_grant_table_op(GNTTABOP_setup_table, &setup, 1);
    gnttab_table = map_frames(frames, NR_GRANT_FRAMES);
    printk("gnttab_table mapped at %p.\n", gnttab_table);
}