multicalls.c

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/*
 * Xen hypercall batching.
 *
 * Xen allows multiple hypercalls to be issued at once, using the
 * multicall interface.  This allows the cost of trapping into the
 * hypervisor to be amortized over several calls.
 *
 * This file implements a simple interface for multicalls.  There's a
 * per-cpu buffer of outstanding multicalls.  When you want to queue a
 * multicall for issuing, you can allocate a multicall slot for the
 * call and its arguments, along with storage for space which is
 * pointed to by the arguments (for passing pointers to structures,
 * etc).  When the multicall is actually issued, all the space for the
 * commands and allocated memory is freed for reuse.
 *
 * Multicalls are flushed whenever any of the buffers get full, or
 * when explicitly requested.  There's no way to get per-multicall
 * return results back.  It will BUG if any of the multicalls fail.
 *
 * Jeremy Fitzhardinge <[email protected]>, XenSource Inc, 2007
 */
#include <linux/percpu.h>
#include <linux/hardirq.h>
#include <linux/debugfs.h>

#include <asm/xen/hypercall.h>

#include "multicalls.h"
#include "debugfs.h"

#define MC_BATCH    32

#define MC_DEBUG    0

#define MC_ARGS     (MC_BATCH * 16)


struct mc_buffer {
    unsigned mcidx, argidx, cbidx;
    struct multicall_entry entries[MC_BATCH];
#if MC_DEBUG
    struct multicall_entry debug[MC_BATCH];
    void *caller[MC_BATCH];
#endif
    unsigned char args[MC_ARGS];
    struct callback {
        void (*fn)(void *);
        void *data;
    } callbacks[MC_BATCH];
};

static DEFINE_PER_CPU(struct mc_buffer, mc_buffer);
DEFINE_PER_CPU(unsigned long, xen_mc_irq_flags);

void xen_mc_flush(void)
{
    struct mc_buffer *b = &__get_cpu_var(mc_buffer);
    struct multicall_entry *mc;
    int ret = 0;
    unsigned long flags;
    int i;

    BUG_ON(preemptible());

    /* Disable interrupts in case someone comes in and queues
       something in the middle */
    local_irq_save(flags);

    trace_xen_mc_flush(b->mcidx, b->argidx, b->cbidx);

    switch (b->mcidx) {
    case 0:
        /* no-op */
        BUG_ON(b->argidx != 0);
        break;

    case 1:
        /* Singleton multicall - bypass multicall machinery
           and just do the call directly. */
        mc = &b->entries[0];

        mc->result = privcmd_call(mc->op,
                      mc->args[0], mc->args[1], mc->args[2], 
                      mc->args[3], mc->args[4]);
        ret = mc->result < 0;
        break;

    default:
#if MC_DEBUG
        memcpy(b->debug, b->entries,
               b->mcidx * sizeof(struct multicall_entry));
#endif

        if (HYPERVISOR_multicall(b->entries, b->mcidx) != 0)
            BUG();
        for (i = 0; i < b->mcidx; i++)
            if (b->entries[i].result < 0)
                ret++;

#if MC_DEBUG
        if (ret) {
            printk(KERN_ERR "%d multicall(s) failed: cpu %d\n",
                   ret, smp_processor_id());
            dump_stack();
            for (i = 0; i < b->mcidx; i++) {
                printk(KERN_DEBUG "  call %2d/%d: op=%lu arg=[%lx] result=%ld\t%pF\n",
                       i+1, b->mcidx,
                       b->debug[i].op,
                       b->debug[i].args[0],
                       b->entries[i].result,
                       b->caller[i]);
            }
        }
#endif
    }

    b->mcidx = 0;
    b->argidx = 0;

    for (i = 0; i < b->cbidx; i++) {
        struct callback *cb = &b->callbacks[i];

        (*cb->fn)(cb->data);
    }
    b->cbidx = 0;

    local_irq_restore(flags);

    WARN_ON(ret);
}

struct multicall_space __xen_mc_entry(size_t args)
{
    struct mc_buffer *b = &__get_cpu_var(mc_buffer);
    struct multicall_space ret;
    unsigned argidx = roundup(b->argidx, sizeof(u64));

    trace_xen_mc_entry_alloc(args);

    BUG_ON(preemptible());
    BUG_ON(b->argidx >= MC_ARGS);

    if (unlikely(b->mcidx == MC_BATCH ||
             (argidx + args) >= MC_ARGS)) {
        trace_xen_mc_flush_reason((b->mcidx == MC_BATCH) ?
                      XEN_MC_FL_BATCH : XEN_MC_FL_ARGS);
        xen_mc_flush();
        argidx = roundup(b->argidx, sizeof(u64));
    }

    ret.mc = &b->entries[b->mcidx];
#if MC_DEBUG
    b->caller[b->mcidx] = __builtin_return_address(0);
#endif
    b->mcidx++;
    ret.args = &b->args[argidx];
    b->argidx = argidx + args;

    BUG_ON(b->argidx >= MC_ARGS);
    return ret;
}

struct multicall_space xen_mc_extend_args(unsigned long op, size_t size)
{
    struct mc_buffer *b = &__get_cpu_var(mc_buffer);
    struct multicall_space ret = { NULL, NULL };

    BUG_ON(preemptible());
    BUG_ON(b->argidx >= MC_ARGS);

    if (unlikely(b->mcidx == 0 ||
             b->entries[b->mcidx - 1].op != op)) {
        trace_xen_mc_extend_args(op, size, XEN_MC_XE_BAD_OP);
        goto out;
    }

    if (unlikely((b->argidx + size) >= MC_ARGS)) {
        trace_xen_mc_extend_args(op, size, XEN_MC_XE_NO_SPACE);
        goto out;
    }

    ret.mc = &b->entries[b->mcidx - 1];
    ret.args = &b->args[b->argidx];
    b->argidx += size;

    BUG_ON(b->argidx >= MC_ARGS);

    trace_xen_mc_extend_args(op, size, XEN_MC_XE_OK);
out:
    return ret;
}

void xen_mc_callback(void (*fn)(void *), void *data)
{
    struct mc_buffer *b = &__get_cpu_var(mc_buffer);
    struct callback *cb;

    if (b->cbidx == MC_BATCH) {
        trace_xen_mc_flush_reason(XEN_MC_FL_CALLBACK);
        xen_mc_flush();
    }

    trace_xen_mc_callback(fn, data);

    cb = &b->callbacks[b->cbidx++];
    cb->fn = fn;
    cb->data = data;
}