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multicalls.c

/*
 * 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 <jeremy@xensource.com>, XenSource Inc, 2007
 */
#include <linux/percpu.h>
#include <linux/hardirq.h>

#include <asm/xen/hypercall.h>

#include "multicalls.h"

#define MC_DEBUG  1

#define MC_BATCH  32
#define MC_ARGS         (MC_BATCH * 16 / sizeof(u64))

struct mc_buffer {
      struct multicall_entry entries[MC_BATCH];
#if MC_DEBUG
      struct multicall_entry debug[MC_BATCH];
#endif
      u64 args[MC_ARGS];
      struct callback {
            void (*fn)(void *);
            void *data;
      } callbacks[MC_BATCH];
      unsigned mcidx, argidx, cbidx;
};

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);
      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);

      if (b->mcidx) {
#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());
                  for(i = 0; i < b->mcidx; i++) {
                        printk("  call %2d/%d: op=%lu arg=[%lx] result=%ld\n",
                               i+1, b->mcidx,
                               b->debug[i].op,
                               b->debug[i].args[0],
                               b->entries[i].result);
                  }
            }
#endif

            b->mcidx = 0;
            b->argidx = 0;
      } else
            BUG_ON(b->argidx != 0);

      local_irq_restore(flags);

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

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

      BUG_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 argspace = (args + sizeof(u64) - 1) / sizeof(u64);

      BUG_ON(preemptible());
      BUG_ON(argspace > MC_ARGS);

      if (b->mcidx == MC_BATCH ||
          (b->argidx + argspace) > MC_ARGS)
            xen_mc_flush();

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

      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)
            xen_mc_flush();

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

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