extras/mini-os/fs-front.c


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/******************************************************************************
 * fs-front.c
 * 
 * Frontend driver for FS split device driver.
 *
 * Copyright (c) 2007, Grzegorz Milos, Sun Microsystems, Inc.
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER 
 * DEALINGS IN THE SOFTWARE.
 */

#undef NDEBUG
#include <os.h>
#include <list.h>
#include <xmalloc.h>
#include <xenbus.h>
#include <gnttab.h>
#include <events.h>
#include <xen/io/fsif.h>
#include <fs.h>
#include <sched.h>

#define preempt_disable()
#define preempt_enable()
#define cmpxchg(p,o,n) synch_cmpxchg(p,o,n)


#ifdef FS_DEBUG
#define DEBUG(_f, _a...) \
    printk("MINI_OS(file=fs-front.c, line=%d) " _f "\n", __LINE__, ## _a)
#else
#define DEBUG(_f, _a...)    ((void)0)
#endif


struct fs_request;
struct fs_import *fs_import;

/******************************************************************************/
/*                      RING REQUEST/RESPONSES HANDLING                       */
/******************************************************************************/

struct fs_request
{
    void *page;
    grant_ref_t gref;
    struct thread *thread;                 /* Thread blocked on this request */
    struct fsif_response shadow_rsp;       /* Response copy writen by the 
                                              interrupt handler */  
};

/* Ring operations:
 * FSIF ring is used differently to Linux-like split devices. This stems from 
 * the fact that no I/O request queue is present. The use of some of the macros
 * defined in ring.h is not allowed, in particular:
 * RING_PUSH_REQUESTS_AND_CHECK_NOTIFY cannot be used.
 *
 * The protocol used for FSIF ring is described below:
 *
 * In order to reserve a request the frontend:
 * a) saves current frontend_ring->req_prod_pvt into a local variable
 * b) checks that there are free request using the local req_prod_pvt
 * c) tries to reserve the request using cmpxchg on frontend_ring->req_prod_pvt
 *    if cmpxchg fails, it means that someone reserved the request, start from
 *    a)
 * 
 * In order to commit a request to the shared ring:
 * a) cmpxchg shared_ring->req_prod from local req_prod_pvt to req_prod_pvt+1 
 *    Loop if unsuccessful.
 * NOTE: Request should be commited to the shared ring as quickly as possible,
 *       because otherwise other threads might busy loop trying to commit next
 *       requests. It also follows that preemption should be disabled, if
 *       possible, for the duration of the request construction.
 */

/* Number of free requests (for use on front side only). */
#define FS_RING_FREE_REQUESTS(_r, _req_prod_pvt)                         \
    (RING_SIZE(_r) - (_req_prod_pvt - (_r)->rsp_cons))


static RING_IDX reserve_fsif_request(struct fs_import *import)
{
    RING_IDX idx; 

    down(&import->reqs_sem);
    preempt_disable();
again:    
    /* We will attempt to reserve slot idx */
    idx = import->ring.req_prod_pvt;
    ASSERT (FS_RING_FREE_REQUESTS(&import->ring, idx));
    /* Attempt to reserve */
    if(cmpxchg(&import->ring.req_prod_pvt, idx, idx+1) != idx)
        goto again;

    return idx; 
}

static void commit_fsif_request(struct fs_import *import, RING_IDX idx)
{
    while(cmpxchg(&import->ring.sring->req_prod, idx, idx+1) != idx)
    {
        printk("Failed to commit a request: req_prod=%d, idx=%d\n",
                import->ring.sring->req_prod, idx);
    }
    preempt_enable();

    /* NOTE: we cannot do anything clever about rsp_event, to hold off
     * notifications, because we don't know if we are a single request (in which
     * case we have to notify always), or a part of a larger request group
     * (when, in some cases, notification isn't required) */
    notify_remote_via_evtchn(import->local_port);
}


static inline void add_id_to_freelist(unsigned int id,unsigned short* freelist)