Linux 3.12.39

[linux/fpc-iii.git] / drivers / hv / channel_mgmt.c

/*
 * Copyright (c) 2009, Microsoft Corporation.
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms and conditions of the GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; if not, write to the Free Software Foundation, Inc., 59 Temple
 * Place - Suite 330, Boston, MA 02111-1307 USA.
 *
 * Authors:
 *   Haiyang Zhang <haiyangz@microsoft.com>
 *   Hank Janssen  <hjanssen@microsoft.com>
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/wait.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/module.h>
#include <linux/completion.h>
#include <linux/hyperv.h>

#include "hyperv_vmbus.h"

struct vmbus_channel_message_table_entry {
        enum vmbus_channel_message_type message_type;
        void (*message_handler)(struct vmbus_channel_message_header *msg);
};


/**
 * vmbus_prep_negotiate_resp() - Create default response for Hyper-V Negotiate message
 * @icmsghdrp: Pointer to msg header structure
 * @icmsg_negotiate: Pointer to negotiate message structure
 * @buf: Raw buffer channel data
 *
 * @icmsghdrp is of type &struct icmsg_hdr.
 * @negop is of type &struct icmsg_negotiate.
 * Set up and fill in default negotiate response message.
 *
 * The fw_version specifies the  framework version that
 * we can support and srv_version specifies the service
 * version we can support.
 *
 * Mainly used by Hyper-V drivers.
 */
bool vmbus_prep_negotiate_resp(struct icmsg_hdr *icmsghdrp,
                                struct icmsg_negotiate *negop, u8 *buf,
                                int fw_version, int srv_version)
{
        int icframe_major, icframe_minor;
        int icmsg_major, icmsg_minor;
        int fw_major, fw_minor;
        int srv_major, srv_minor;
        int i;
        bool found_match = false;

        icmsghdrp->icmsgsize = 0x10;
        fw_major = (fw_version >> 16);
        fw_minor = (fw_version & 0xFFFF);

        srv_major = (srv_version >> 16);
        srv_minor = (srv_version & 0xFFFF);

        negop = (struct icmsg_negotiate *)&buf[
                sizeof(struct vmbuspipe_hdr) +
                sizeof(struct icmsg_hdr)];

        icframe_major = negop->icframe_vercnt;
        icframe_minor = 0;

        icmsg_major = negop->icmsg_vercnt;
        icmsg_minor = 0;

        /*
         * Select the framework version number we will
         * support.
         */

        for (i = 0; i < negop->icframe_vercnt; i++) {
                if ((negop->icversion_data[i].major == fw_major) &&
                   (negop->icversion_data[i].minor == fw_minor)) {
                        icframe_major = negop->icversion_data[i].major;
                        icframe_minor = negop->icversion_data[i].minor;
                        found_match = true;
                }
        }

        if (!found_match)
                goto fw_error;

        found_match = false;

        for (i = negop->icframe_vercnt;
                 (i < negop->icframe_vercnt + negop->icmsg_vercnt); i++) {
                if ((negop->icversion_data[i].major == srv_major) &&
                   (negop->icversion_data[i].minor == srv_minor)) {
                        icmsg_major = negop->icversion_data[i].major;
                        icmsg_minor = negop->icversion_data[i].minor;
                        found_match = true;
                }
        }

        /*
         * Respond with the framework and service
         * version numbers we can support.
         */

fw_error:
        if (!found_match) {
                negop->icframe_vercnt = 0;
                negop->icmsg_vercnt = 0;
        } else {
                negop->icframe_vercnt = 1;
                negop->icmsg_vercnt = 1;
        }

        negop->icversion_data[0].major = icframe_major;
        negop->icversion_data[0].minor = icframe_minor;
        negop->icversion_data[1].major = icmsg_major;
        negop->icversion_data[1].minor = icmsg_minor;
        return found_match;
}

EXPORT_SYMBOL_GPL(vmbus_prep_negotiate_resp);

/*
 * alloc_channel - Allocate and initialize a vmbus channel object
 */
static struct vmbus_channel *alloc_channel(void)
{
        struct vmbus_channel *channel;

        channel = kzalloc(sizeof(*channel), GFP_ATOMIC);
        if (!channel)
                return NULL;

        spin_lock_init(&channel->inbound_lock);
        spin_lock_init(&channel->sc_lock);

        INIT_LIST_HEAD(&channel->sc_list);

        channel->controlwq = create_workqueue("hv_vmbus_ctl");
        if (!channel->controlwq) {
                kfree(channel);
                return NULL;
        }

        return channel;
}

/*
 * release_hannel - Release the vmbus channel object itself
 */
static void release_channel(struct work_struct *work)
{
        struct vmbus_channel *channel = container_of(work,
                                                     struct vmbus_channel,
                                                     work);

        destroy_workqueue(channel->controlwq);

        kfree(channel);
}

/*
 * free_channel - Release the resources used by the vmbus channel object
 */
static void free_channel(struct vmbus_channel *channel)
{

        /*
         * We have to release the channel's workqueue/thread in the vmbus's
         * workqueue/thread context
         * ie we can't destroy ourselves.
         */
        INIT_WORK(&channel->work, release_channel);
        queue_work(vmbus_connection.work_queue, &channel->work);
}



/*
 * vmbus_process_rescind_offer -
 * Rescind the offer by initiating a device removal
 */
static void vmbus_process_rescind_offer(struct work_struct *work)
{
        struct vmbus_channel *channel = container_of(work,
                                                     struct vmbus_channel,
                                                     work);
        unsigned long flags;
        struct vmbus_channel *primary_channel;
        struct vmbus_channel_relid_released msg;
        struct device *dev;

        if (channel->device_obj) {
                dev = get_device(&channel->device_obj->device);
                if (dev) {
                        vmbus_device_unregister(channel->device_obj);
                        put_device(dev);
                }
        }

        memset(&msg, 0, sizeof(struct vmbus_channel_relid_released));
        msg.child_relid = channel->offermsg.child_relid;
        msg.header.msgtype = CHANNELMSG_RELID_RELEASED;
        vmbus_post_msg(&msg, sizeof(struct vmbus_channel_relid_released));

        if (channel->primary_channel == NULL) {
                spin_lock_irqsave(&vmbus_connection.channel_lock, flags);
                list_del(&channel->listentry);
                spin_unlock_irqrestore(&vmbus_connection.channel_lock, flags);
        } else {
                primary_channel = channel->primary_channel;
                spin_lock_irqsave(&primary_channel->sc_lock, flags);
                list_del(&channel->sc_list);
                spin_unlock_irqrestore(&primary_channel->sc_lock, flags);
        }
        free_channel(channel);
}

void vmbus_free_channels(void)
{
        struct vmbus_channel *channel;

        list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
                vmbus_device_unregister(channel->device_obj);
                kfree(channel->device_obj);
                free_channel(channel);
        }
}

/*
 * vmbus_process_offer - Process the offer by creating a channel/device
 * associated with this offer
 */
static void vmbus_process_offer(struct work_struct *work)
{
        struct vmbus_channel *newchannel = container_of(work,
                                                        struct vmbus_channel,
                                                        work);
        struct vmbus_channel *channel;
        bool fnew = true;
        int ret;
        unsigned long flags;

        /* The next possible work is rescind handling */
        INIT_WORK(&newchannel->work, vmbus_process_rescind_offer);

        /* Make sure this is a new offer */
        spin_lock_irqsave(&vmbus_connection.channel_lock, flags);

        list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
                if (!uuid_le_cmp(channel->offermsg.offer.if_type,
                        newchannel->offermsg.offer.if_type) &&
                        !uuid_le_cmp(channel->offermsg.offer.if_instance,
                                newchannel->offermsg.offer.if_instance)) {
                        fnew = false;
                        break;
                }
        }

        if (fnew)
                list_add_tail(&newchannel->listentry,
                              &vmbus_connection.chn_list);

        spin_unlock_irqrestore(&vmbus_connection.channel_lock, flags);

        if (!fnew) {
                /*
                 * Check to see if this is a sub-channel.
                 */
                if (newchannel->offermsg.offer.sub_channel_index != 0) {
                        /*
                         * Process the sub-channel.
                         */
                        newchannel->primary_channel = channel;
                        spin_lock_irqsave(&channel->sc_lock, flags);
                        list_add_tail(&newchannel->sc_list, &channel->sc_list);
                        spin_unlock_irqrestore(&channel->sc_lock, flags);
                        newchannel->state = CHANNEL_OPEN_STATE;
                        if (channel->sc_creation_callback != NULL)
                                channel->sc_creation_callback(newchannel);

                        return;
                }

                free_channel(newchannel);
                return;
        }

        /*
         * This state is used to indicate a successful open
         * so that when we do close the channel normally, we
         * can cleanup properly
         */
        newchannel->state = CHANNEL_OPEN_STATE;

        /*
         * Start the process of binding this offer to the driver
         * We need to set the DeviceObject field before calling
         * vmbus_child_dev_add()
         */
        newchannel->device_obj = vmbus_device_create(
                &newchannel->offermsg.offer.if_type,
                &newchannel->offermsg.offer.if_instance,
                newchannel);

        /*
         * Add the new device to the bus. This will kick off device-driver
         * binding which eventually invokes the device driver's AddDevice()
         * method.
         */
        ret = vmbus_device_register(newchannel->device_obj);
        if (ret != 0) {
                pr_err("unable to add child device object (relid %d)\n",
                           newchannel->offermsg.child_relid);

                spin_lock_irqsave(&vmbus_connection.channel_lock, flags);
                list_del(&newchannel->listentry);
                spin_unlock_irqrestore(&vmbus_connection.channel_lock, flags);
                kfree(newchannel->device_obj);

                free_channel(newchannel);
        }
}

enum {
        IDE = 0,
        SCSI,
        NIC,
        MAX_PERF_CHN,
};

/*
 * This is an array of device_ids (device types) that are performance critical.
 * We attempt to distribute the interrupt load for these devices across
 * all available CPUs.
 */
static const struct hv_vmbus_device_id hp_devs[] = {
        /* IDE */
        { HV_IDE_GUID, },
        /* Storage - SCSI */
        { HV_SCSI_GUID, },
        /* Network */
        { HV_NIC_GUID, },
};


/*
 * We use this state to statically distribute the channel interrupt load.
 */
static u32  next_vp;

/*
 * Starting with Win8, we can statically distribute the incoming
 * channel interrupt load by binding a channel to VCPU. We
 * implement here a simple round robin scheme for distributing
 * the interrupt load.
 * We will bind channels that are not performance critical to cpu 0 and
 * performance critical channels (IDE, SCSI and Network) will be uniformly
 * distributed across all available CPUs.
 */
static u32 get_vp_index(uuid_le *type_guid)
{
        u32 cur_cpu;
        int i;
        bool perf_chn = false;
        u32 max_cpus = num_online_cpus();

        for (i = IDE; i < MAX_PERF_CHN; i++) {
                if (!memcmp(type_guid->b, hp_devs[i].guid,
                                 sizeof(uuid_le))) {
                        perf_chn = true;
                        break;
                }
        }
        if ((vmbus_proto_version == VERSION_WS2008) ||
            (vmbus_proto_version == VERSION_WIN7) || (!perf_chn)) {
                /*
                 * Prior to win8, all channel interrupts are
                 * delivered on cpu 0.
                 * Also if the channel is not a performance critical
                 * channel, bind it to cpu 0.
                 */
                return 0;
        }
        cur_cpu = (++next_vp % max_cpus);
        return hv_context.vp_index[cur_cpu];
}

/*
 * vmbus_onoffer - Handler for channel offers from vmbus in parent partition.
 *
 */
static void vmbus_onoffer(struct vmbus_channel_message_header *hdr)
{
        struct vmbus_channel_offer_channel *offer;
        struct vmbus_channel *newchannel;

        offer = (struct vmbus_channel_offer_channel *)hdr;

        /* Allocate the channel object and save this offer. */
        newchannel = alloc_channel();
        if (!newchannel) {
                pr_err("Unable to allocate channel object\n");
                return;
        }

        /*
         * By default we setup state to enable batched
         * reading. A specific service can choose to
         * disable this prior to opening the channel.
         */
        newchannel->batched_reading = true;

        /*
         * Setup state for signalling the host.
         */
        newchannel->sig_event = (struct hv_input_signal_event *)
                                (ALIGN((unsigned long)
                                &newchannel->sig_buf,
                                HV_HYPERCALL_PARAM_ALIGN));

        newchannel->sig_event->connectionid.asu32 = 0;
        newchannel->sig_event->connectionid.u.id = VMBUS_EVENT_CONNECTION_ID;
        newchannel->sig_event->flag_number = 0;
        newchannel->sig_event->rsvdz = 0;

        if (vmbus_proto_version != VERSION_WS2008) {
                newchannel->is_dedicated_interrupt =
                                (offer->is_dedicated_interrupt != 0);
                newchannel->sig_event->connectionid.u.id =
                                offer->connection_id;
        }

        newchannel->target_vp = get_vp_index(&offer->offer.if_type);

        memcpy(&newchannel->offermsg, offer,
               sizeof(struct vmbus_channel_offer_channel));
        newchannel->monitor_grp = (u8)offer->monitorid / 32;
        newchannel->monitor_bit = (u8)offer->monitorid % 32;

        INIT_WORK(&newchannel->work, vmbus_process_offer);
        queue_work(newchannel->controlwq, &newchannel->work);
}

/*
 * vmbus_onoffer_rescind - Rescind offer handler.
 *
 * We queue a work item to process this offer synchronously
 */
static void vmbus_onoffer_rescind(struct vmbus_channel_message_header *hdr)
{
        struct vmbus_channel_rescind_offer *rescind;
        struct vmbus_channel *channel;

        rescind = (struct vmbus_channel_rescind_offer *)hdr;
        channel = relid2channel(rescind->child_relid);

        if (channel == NULL)
                /* Just return here, no channel found */
                return;

        /* work is initialized for vmbus_process_rescind_offer() from
         * vmbus_process_offer() where the channel got created */
        queue_work(channel->controlwq, &channel->work);
}

/*
 * vmbus_onoffers_delivered -
 * This is invoked when all offers have been delivered.
 *
 * Nothing to do here.
 */
static void vmbus_onoffers_delivered(
                        struct vmbus_channel_message_header *hdr)
{
}

/*
 * vmbus_onopen_result - Open result handler.
 *
 * This is invoked when we received a response to our channel open request.
 * Find the matching request, copy the response and signal the requesting
 * thread.
 */
static void vmbus_onopen_result(struct vmbus_channel_message_header *hdr)
{
        struct vmbus_channel_open_result *result;
        struct vmbus_channel_msginfo *msginfo;
        struct vmbus_channel_message_header *requestheader;
        struct vmbus_channel_open_channel *openmsg;
        unsigned long flags;

        result = (struct vmbus_channel_open_result *)hdr;

        /*
         * Find the open msg, copy the result and signal/unblock the wait event
         */
        spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);

        list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
                                msglistentry) {
                requestheader =
                        (struct vmbus_channel_message_header *)msginfo->msg;

                if (requestheader->msgtype == CHANNELMSG_OPENCHANNEL) {
                        openmsg =
                        (struct vmbus_channel_open_channel *)msginfo->msg;
                        if (openmsg->child_relid == result->child_relid &&
                            openmsg->openid == result->openid) {
                                memcpy(&msginfo->response.open_result,
                                       result,
                                       sizeof(
                                        struct vmbus_channel_open_result));
                                complete(&msginfo->waitevent);
                                break;
                        }
                }
        }
        spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

/*
 * vmbus_ongpadl_created - GPADL created handler.
 *
 * This is invoked when we received a response to our gpadl create request.
 * Find the matching request, copy the response and signal the requesting
 * thread.
 */
static void vmbus_ongpadl_created(struct vmbus_channel_message_header *hdr)
{
        struct vmbus_channel_gpadl_created *gpadlcreated;
        struct vmbus_channel_msginfo *msginfo;
        struct vmbus_channel_message_header *requestheader;
        struct vmbus_channel_gpadl_header *gpadlheader;
        unsigned long flags;

        gpadlcreated = (struct vmbus_channel_gpadl_created *)hdr;

        /*
         * Find the establish msg, copy the result and signal/unblock the wait
         * event
         */
        spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);

        list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
                                msglistentry) {
                requestheader =
                        (struct vmbus_channel_message_header *)msginfo->msg;

                if (requestheader->msgtype == CHANNELMSG_GPADL_HEADER) {
                        gpadlheader =
                        (struct vmbus_channel_gpadl_header *)requestheader;

                        if ((gpadlcreated->child_relid ==
                             gpadlheader->child_relid) &&
                            (gpadlcreated->gpadl == gpadlheader->gpadl)) {
                                memcpy(&msginfo->response.gpadl_created,
                                       gpadlcreated,
                                       sizeof(
                                        struct vmbus_channel_gpadl_created));
                                complete(&msginfo->waitevent);
                                break;
                        }
                }
        }
        spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

/*
 * vmbus_ongpadl_torndown - GPADL torndown handler.
 *
 * This is invoked when we received a response to our gpadl teardown request.
 * Find the matching request, copy the response and signal the requesting
 * thread.
 */
static void vmbus_ongpadl_torndown(
                        struct vmbus_channel_message_header *hdr)
{
        struct vmbus_channel_gpadl_torndown *gpadl_torndown;
        struct vmbus_channel_msginfo *msginfo;
        struct vmbus_channel_message_header *requestheader;
        struct vmbus_channel_gpadl_teardown *gpadl_teardown;
        unsigned long flags;

        gpadl_torndown = (struct vmbus_channel_gpadl_torndown *)hdr;

        /*
         * Find the open msg, copy the result and signal/unblock the wait event
         */
        spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);

        list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
                                msglistentry) {
                requestheader =
                        (struct vmbus_channel_message_header *)msginfo->msg;

                if (requestheader->msgtype == CHANNELMSG_GPADL_TEARDOWN) {
                        gpadl_teardown =
                        (struct vmbus_channel_gpadl_teardown *)requestheader;

                        if (gpadl_torndown->gpadl == gpadl_teardown->gpadl) {
                                memcpy(&msginfo->response.gpadl_torndown,
                                       gpadl_torndown,
                                       sizeof(
                                        struct vmbus_channel_gpadl_torndown));
                                complete(&msginfo->waitevent);
                                break;
                        }
                }
        }
        spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

/*
 * vmbus_onversion_response - Version response handler
 *
 * This is invoked when we received a response to our initiate contact request.
 * Find the matching request, copy the response and signal the requesting
 * thread.
 */
static void vmbus_onversion_response(
                struct vmbus_channel_message_header *hdr)
{
        struct vmbus_channel_msginfo *msginfo;
        struct vmbus_channel_message_header *requestheader;
        struct vmbus_channel_version_response *version_response;
        unsigned long flags;

        version_response = (struct vmbus_channel_version_response *)hdr;
        spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);

        list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
                                msglistentry) {
                requestheader =
                        (struct vmbus_channel_message_header *)msginfo->msg;

                if (requestheader->msgtype ==
                    CHANNELMSG_INITIATE_CONTACT) {
                        memcpy(&msginfo->response.version_response,
                              version_response,
                              sizeof(struct vmbus_channel_version_response));
                        complete(&msginfo->waitevent);
                }
        }
        spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

/* Channel message dispatch table */
static struct vmbus_channel_message_table_entry
        channel_message_table[CHANNELMSG_COUNT] = {
        {CHANNELMSG_INVALID,                    NULL},
        {CHANNELMSG_OFFERCHANNEL,               vmbus_onoffer},
        {CHANNELMSG_RESCIND_CHANNELOFFER,       vmbus_onoffer_rescind},
        {CHANNELMSG_REQUESTOFFERS,              NULL},
        {CHANNELMSG_ALLOFFERS_DELIVERED,        vmbus_onoffers_delivered},
        {CHANNELMSG_OPENCHANNEL,                NULL},
        {CHANNELMSG_OPENCHANNEL_RESULT, vmbus_onopen_result},
        {CHANNELMSG_CLOSECHANNEL,               NULL},
        {CHANNELMSG_GPADL_HEADER,               NULL},
        {CHANNELMSG_GPADL_BODY,         NULL},
        {CHANNELMSG_GPADL_CREATED,              vmbus_ongpadl_created},
        {CHANNELMSG_GPADL_TEARDOWN,             NULL},
        {CHANNELMSG_GPADL_TORNDOWN,             vmbus_ongpadl_torndown},
        {CHANNELMSG_RELID_RELEASED,             NULL},
        {CHANNELMSG_INITIATE_CONTACT,           NULL},
        {CHANNELMSG_VERSION_RESPONSE,           vmbus_onversion_response},
        {CHANNELMSG_UNLOAD,                     NULL},
};

/*
 * vmbus_onmessage - Handler for channel protocol messages.
 *
 * This is invoked in the vmbus worker thread context.
 */
void vmbus_onmessage(void *context)
{
        struct hv_message *msg = context;
        struct vmbus_channel_message_header *hdr;
        int size;

        hdr = (struct vmbus_channel_message_header *)msg->u.payload;
        size = msg->header.payload_size;

        if (hdr->msgtype >= CHANNELMSG_COUNT) {
                pr_err("Received invalid channel message type %d size %d\n",
                           hdr->msgtype, size);
                print_hex_dump_bytes("", DUMP_PREFIX_NONE,
                                     (unsigned char *)msg->u.payload, size);
                return;
        }

        if (channel_message_table[hdr->msgtype].message_handler)
                channel_message_table[hdr->msgtype].message_handler(hdr);
        else
                pr_err("Unhandled channel message type %d\n", hdr->msgtype);
}

/*
 * vmbus_request_offers - Send a request to get all our pending offers.
 */
int vmbus_request_offers(void)
{
        struct vmbus_channel_message_header *msg;
        struct vmbus_channel_msginfo *msginfo;
        int ret, t;

        msginfo = kmalloc(sizeof(*msginfo) +
                          sizeof(struct vmbus_channel_message_header),
                          GFP_KERNEL);
        if (!msginfo)
                return -ENOMEM;

        init_completion(&msginfo->waitevent);

        msg = (struct vmbus_channel_message_header *)msginfo->msg;

        msg->msgtype = CHANNELMSG_REQUESTOFFERS;


        ret = vmbus_post_msg(msg,
                               sizeof(struct vmbus_channel_message_header));
        if (ret != 0) {
                pr_err("Unable to request offers - %d\n", ret);

                goto cleanup;
        }

        t = wait_for_completion_timeout(&msginfo->waitevent, 5*HZ);
        if (t == 0) {
                ret = -ETIMEDOUT;
                goto cleanup;
        }



cleanup:
        kfree(msginfo);

        return ret;
}

/*
 * Retrieve the (sub) channel on which to send an outgoing request.
 * When a primary channel has multiple sub-channels, we choose a
 * channel whose VCPU binding is closest to the VCPU on which
 * this call is being made.
 */
struct vmbus_channel *vmbus_get_outgoing_channel(struct vmbus_channel *primary)
{
        struct list_head *cur, *tmp;
        int cur_cpu = hv_context.vp_index[smp_processor_id()];
        struct vmbus_channel *cur_channel;
        struct vmbus_channel *outgoing_channel = primary;
        int cpu_distance, new_cpu_distance;

        if (list_empty(&primary->sc_list))
                return outgoing_channel;

        list_for_each_safe(cur, tmp, &primary->sc_list) {
                cur_channel = list_entry(cur, struct vmbus_channel, sc_list);
                if (cur_channel->state != CHANNEL_OPENED_STATE)
                        continue;

                if (cur_channel->target_vp == cur_cpu)
                        return cur_channel;

                cpu_distance = ((outgoing_channel->target_vp > cur_cpu) ?
                                (outgoing_channel->target_vp - cur_cpu) :
                                (cur_cpu - outgoing_channel->target_vp));

                new_cpu_distance = ((cur_channel->target_vp > cur_cpu) ?
                                (cur_channel->target_vp - cur_cpu) :
                                (cur_cpu - cur_channel->target_vp));

                if (cpu_distance < new_cpu_distance)
                        continue;

                outgoing_channel = cur_channel;
        }

        return outgoing_channel;
}
EXPORT_SYMBOL_GPL(vmbus_get_outgoing_channel);

static void invoke_sc_cb(struct vmbus_channel *primary_channel)
{
        struct list_head *cur, *tmp;
        struct vmbus_channel *cur_channel;

        if (primary_channel->sc_creation_callback == NULL)
                return;

        list_for_each_safe(cur, tmp, &primary_channel->sc_list) {
                cur_channel = list_entry(cur, struct vmbus_channel, sc_list);

                primary_channel->sc_creation_callback(cur_channel);
        }
}

void vmbus_set_sc_create_callback(struct vmbus_channel *primary_channel,
                                void (*sc_cr_cb)(struct vmbus_channel *new_sc))
{
        primary_channel->sc_creation_callback = sc_cr_cb;
}
EXPORT_SYMBOL_GPL(vmbus_set_sc_create_callback);

bool vmbus_are_subchannels_present(struct vmbus_channel *primary)
{
        bool ret;

        ret = !list_empty(&primary->sc_list);

        if (ret) {
                /*
                 * Invoke the callback on sub-channel creation.
                 * This will present a uniform interface to the
                 * clients.
                 */
                invoke_sc_cb(primary);
        }

        return ret;
}
EXPORT_SYMBOL_GPL(vmbus_are_subchannels_present);