block: add -drive copy-on-read=on|off

This patch adds the -drive copy-on-read=on|off command-line option:

  copy-on-read=on|off
  copy-on-read is "on" or "off" and enables whether to copy read backing
  file sectors into the image file.  Copy-on-read avoids accessing the
  same backing file sectors repeatedly and is useful when the backing
  file is over a slow network.  By default copy-on-read is off.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -257,6 +257,7 @@ DriveInfo *drive_init(QemuOpts *opts, int default_to_scsi)
     DriveInfo *dinfo;
     BlockIOLimit io_limits;
     int snapshot = 0;
+    bool copy_on_read;
     int ret;
 
     translation = BIOS_ATA_TRANSLATION_AUTO;
@@ -273,6 +274,7 @@ DriveInfo *drive_init(QemuOpts *opts, int default_to_scsi)
 
     snapshot = qemu_opt_get_bool(opts, "snapshot", 0);
     ro = qemu_opt_get_bool(opts, "readonly", 0);
+    copy_on_read = qemu_opt_get_bool(opts, "copy-on-read", false);
 
     file = qemu_opt_get(opts, "file");
     serial = qemu_opt_get(opts, "serial");
@@ -546,6 +548,10 @@ DriveInfo *drive_init(QemuOpts *opts, int default_to_scsi)
         bdrv_flags |= (BDRV_O_SNAPSHOT|BDRV_O_CACHE_WB|BDRV_O_NO_FLUSH);
     }
 
+    if (copy_on_read) {
+        bdrv_flags |= BDRV_O_COPY_ON_READ;
+    }
+
     if (media == MEDIA_CDROM) {
         /* CDROM is fine for any interface, don't check.  */
         ro = 1;

hmp-commands.hx

@@ -860,9 +860,10 @@ ETEXI
         .args_type  = "pci_addr:s,opts:s",
         .params     = "[[<domain>:]<bus>:]<slot>\n"
                       "[file=file][,if=type][,bus=n]\n"
-                      "[,unit=m][,media=d][index=i]\n"
+                      "[,unit=m][,media=d][,index=i]\n"
                       "[,cyls=c,heads=h,secs=s[,trans=t]]\n"
-                      "[snapshot=on|off][,cache=on|off]",
+                      "[,snapshot=on|off][,cache=on|off]\n"
+                      "[,readonly=on|off][,copy-on-read=on|off]",
         .help       = "add drive to PCI storage controller",
         .mhandler.cmd = drive_hot_add,
     },

qemu-config.c

@@ -109,6 +109,10 @@ static QemuOptsList qemu_drive_opts = {
             .name = "bps_wr",
             .type = QEMU_OPT_NUMBER,
             .help = "limit write bytes per second",
+        },{
+            .name = "copy-on-read",
+            .type = QEMU_OPT_BOOL,
+            .help = "copy read data from backing file into image file",
         },
         { /* end of list */ }
     },

qemu-options.hx

@@ -135,7 +135,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "       [,cyls=c,heads=h,secs=s[,trans=t]][,snapshot=on|off]\n"
     "       [,cache=writethrough|writeback|none|directsync|unsafe][,format=f]\n"
     "       [,serial=s][,addr=A][,id=name][,aio=threads|native]\n"
-    "       [,readonly=on|off]\n"
+    "       [,readonly=on|off][,copy-on-read=on|off]\n"
     "       [[,bps=b]|[[,bps_rd=r][,bps_wr=w]]][[,iops=i]|[[,iops_rd=r][,iops_wr=w]]\n"
     "                use 'file' as a drive image\n", QEMU_ARCH_ALL)
 STEXI
@@ -187,6 +187,9 @@ host disk is full; report the error to the guest otherwise).
 The default setting is @option{werror=enospc} and @option{rerror=report}.
 @item readonly
 Open drive @option{file} as read-only. Guest write attempts will fail.
+@item copy-on-read=@var{copy-on-read}
+@var{copy-on-read} is "on" or "off" and enables whether to copy read backing
+file sectors into the image file.
 @end table
 
 By default, writethrough caching is used for all block device.  This means that
@@ -218,6 +221,10 @@ like your host losing power, the disk storage getting disconnected accidently,
 etc. you're image will most probably be rendered unusable.   When using
 the @option{-snapshot} option, unsafe caching is always used.
 
+Copy-on-read avoids accessing the same backing file sectors repeatedly and is
+useful when the backing file is over a slow network.  By default copy-on-read
+is off.
+
 Instead of @option{-cdrom} you can use:
 @example
 qemu -drive file=file,index=2,media=cdrom