arch/i386/xen/multicalls.c

   1 /*
   2  * Xen hypercall batching.
   3  *
   4  * Xen allows multiple hypercalls to be issued at once, using the
   5  * multicall interface.  This allows the cost of trapping into the
   6  * hypervisor to be amortized over several calls.
   7  *
   8  * This file implements a simple interface for multicalls.  There's a
   9  * per-cpu buffer of outstanding multicalls.  When you want to queue a
  10  * multicall for issuing, you can allocate a multicall slot for the
  11  * call and its arguments, along with storage for space which is
  12  * pointed to by the arguments (for passing pointers to structures,
  13  * etc).  When the multicall is actually issued, all the space for the
  14  * commands and allocated memory is freed for reuse.
  15  *
  16  * Multicalls are flushed whenever any of the buffers get full, or
  17  * when explicitly requested.  There's no way to get per-multicall
  18  * return results back.  It will BUG if any of the multicalls fail.
  19  *
  20  * Jeremy Fitzhardinge <jeremy@xensource.com>, XenSource Inc, 2007
  21  */
  22 #include <linux/percpu.h>
  23 #include <linux/hardirq.h>
  24
  25 #include <asm/xen/hypercall.h>
  26
  27 #include "multicalls.h"
  28
  29 #define MC_BATCH        32
  30 #define MC_ARGS         (MC_BATCH * 16 / sizeof(u64))
  31
  32 struct mc_buffer {
  33         struct multicall_entry entries[MC_BATCH];
  34         u64 args[MC_ARGS];
  35         unsigned mcidx, argidx;
  36 };
  37
  38 static DEFINE_PER_CPU(struct mc_buffer, mc_buffer);
  39 DEFINE_PER_CPU(unsigned long, xen_mc_irq_flags);
  40
  41 void xen_mc_flush(void)
  42 {
  43         struct mc_buffer *b = &__get_cpu_var(mc_buffer);
  44         int ret = 0;
  45         unsigned long flags;
  46
  47         BUG_ON(preemptible());
  48
  49         /* Disable interrupts in case someone comes in and queues
  50            something in the middle */
  51         local_irq_save(flags);
  52
  53         if (b->mcidx) {
  54                 int i;
  55
  56                 if (HYPERVISOR_multicall(b->entries, b->mcidx) != 0)
  57                         BUG();
  58                 for (i = 0; i < b->mcidx; i++)
  59                         if (b->entries[i].result < 0)
  60                                 ret++;
  61                 b->mcidx = 0;
  62                 b->argidx = 0;
  63         } else
  64                 BUG_ON(b->argidx != 0);
  65
  66         local_irq_restore(flags);
  67
  68         BUG_ON(ret);
  69 }
  70
  71 struct multicall_space __xen_mc_entry(size_t args)
  72 {
  73         struct mc_buffer *b = &__get_cpu_var(mc_buffer);
  74         struct multicall_space ret;
  75         unsigned argspace = (args + sizeof(u64) - 1) / sizeof(u64);
  76
  77         BUG_ON(preemptible());
  78         BUG_ON(argspace > MC_ARGS);
  79
  80         if (b->mcidx == MC_BATCH ||
  81             (b->argidx + argspace) > MC_ARGS)
  82                 xen_mc_flush();
  83
  84         ret.mc = &b->entries[b->mcidx];
  85         b->mcidx++;
  86         ret.args = &b->args[b->argidx];
  87         b->argidx += argspace;
  88
  89         return ret;
  90 }