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



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 :-) ?

> 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 :-)

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

Daniel

-- 
Daniel Veillard      | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
daniel veillard com  | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library  http://libvirt.org/


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