[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

Re: [libvirt] [PATCH] migrate VMs between different-endian hosts



On 04/10/2011 11:09 PM, Daniel Veillard wrote:
On Sat, Apr 09, 2011 at 11:48:41AM -0400, Stefan Berger wrote:
This patch enables the migration of Qemu VMs between hosts of
different endianess. I tested this by migrating a i686 VM between a
x86 and ppc64 host.
   OMG, there is really a use case for this :-) ?
:-) There may be other architectures that run more efficiently than an x86 one.
Btw, my use case is 'testing'.
I am converting the 'int's in the VM's state header to uint32_t
assuming this doesn't break compatibility with existing deployments
other than Linux.

Signed-off-by: Stefan Berger<stefanb linux vnet ibm com>

---
  src/qemu/qemu_driver.c |   25 ++++++++++++++++++++-----
  1 file changed, 20 insertions(+), 5 deletions(-)

Index: libvirt-acl/src/qemu/qemu_driver.c
===================================================================
--- libvirt-acl.orig/src/qemu/qemu_driver.c
+++ libvirt-acl/src/qemu/qemu_driver.c
@@ -43,6 +43,7 @@
  #include<sys/wait.h>
  #include<sys/ioctl.h>
  #include<sys/un.h>
+#include<byteswap.h>


  #include "qemu_driver.h"
@@ -1881,13 +1882,22 @@ VIR_ENUM_IMPL(qemudSaveCompression, QEMU

  struct qemud_save_header {
      char magic[sizeof(QEMUD_SAVE_MAGIC)-1];
-    int version;
-    int xml_len;
-    int was_running;
-    int compressed;
-    int unused[15];
+    uint32_t version;
+    uint32_t xml_len;
+    uint32_t was_running;
+    uint32_t compressed;
+    uint32_t unused[15];
  };

+static inline void
+bswap_header(struct qemud_save_header *hdr) {
+    hdr->version = bswap_32(hdr->version);
+    hdr->xml_len = bswap_32(hdr->xml_len);
+    hdr->was_running = bswap_32(hdr->was_running);
+    hdr->compressed = bswap_32(hdr->compressed);
+}
+
+
  /* return -errno on failure, or 0 on success */
  static int
  qemuDomainSaveHeader(int fd, const char *path, char *xml,
@@ -3097,6 +3107,11 @@ qemuDomainSaveImageOpen(struct qemud_dri
      }

      if (header.version>  QEMUD_SAVE_VERSION) {
+        /* convert endianess and try again */
+        bswap_header(&header);
+    }
   Hum, isn't there a more reliable way to detect the change of
endianness ? That's a bit fishy IMHO :-)
The problem is that the header should not have been written in a hosts' native format. So what can go wrong? QEMUD_SAVE_VERSION is '2'. Either we find 1 or 2 here and go ahead and accept it 'as-is'. Otherwise anything bigger than 3 is not accepted and swapped. 3 then becomes 0x03 00 00 00 and is discarded. 0x 02 00 00 00 would be swapped to '2' and accepted.

   Stefan
+    if (header.version>  QEMUD_SAVE_VERSION) {
          qemuReportError(VIR_ERR_OPERATION_FAILED,
                          _("image version is not supported (%d>  %d)"),
                          header.version, QEMUD_SAVE_VERSION);
Daniel



[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]