Re: [Qemu-devel] [PATCH v9 00/22] legacy virtio support for cross-endian targets

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 24, 2014 at 07:06:58PM +0200, Greg Kurz wrote:
> The current legacy virtio devices have a fundamental flaw: they all share
> data between host and guest with guest endianness ordering. This is ok for
> nearly all architectures that have fixed endianness. Unfortunately, it breaks
> for recent PPC64 and ARM targets that can change endianness at runtime.
> The virtio-1.0 specification fixes the issue by enforcing little-endian
> ordering. It may take some time though until the code for 1.0 gets available
> and supported, and all the users can migrate. There have been discussions
> for some monthes about supporting such oddity: now we have little-endian
> PPC64 distros available, it is worth to propose something.
> 
> This patch set brings legacy virtio support for cross-endian targets. The
> rationale is that we add a new device_endianness property to VirtIODevice.
> This property is used as a runtime indicator to decide wether we should
> do little-endian or big-endian conversion, as opposed to the compile time
> choice we have now with TARGTE_WORDS_BIGENDIAN.

typo :) no need to repost for this.

> The choice was made to
> sample the device endianness out of the endianness mode of the guest
> CPU that does the reset. It is an evil but logical consequence of the
> initial flaw in the virtio specification, and it was agreed that the concept
> would be a good common base for ARM and PPC64 enablement at least. Please
> note also that this new property is state and must be preserved across
> migrations.
> 
> There are several parts in the serie:
> - patches 1 and 2 are simple fixes
> - patches 3 to 9 introduce VMState based subsections in the virtio
>   migration code. This is needed because we introduce a new property
>   in VirtIODevice that we want to migrate without ruining compatibility
>   efforts
> - patches 10 to 13 bring virtio device endianness and memory accessors
>   to be used by the virtio code
> - patches 14 to 20 wire the new memory accessors everywhere accross the
>   virtio code
> - patch 21 is the PPC64 enablement
> - patch 22 is a follow-up workaround to disable vhost-net acceleration
>   in the case the host and guest have different endianness, because
>   it is not supported for the moment
> 
> Changes since v8 are provided in each patch.
> 
> Cheers.
> 
> ---
> 
> Alexander Graf (1):
>       virtio-serial: don't migrate the config space
> 
> Cédric Le Goater (1):
>       virtio-net: byteswap virtio-net header
> 
> Greg Kurz (14):
>       virtio: introduce device specific migration calls
>       virtio-net: implement per-device migration calls
>       virtio-blk: implement per-device migration calls
>       virtio-serial: implement per-device migration calls
>       virtio-balloon: implement per-device migration calls
>       virtio-rng: implement per-device migration calls
>       virtio: add subsections to the migration stream
>       exec: introduce target_words_bigendian() helper
>       cpu: introduce CPUClass::virtio_is_big_endian()
>       virtio: add endian-ambivalent support to VirtIODevice
>       virtio: memory accessors for endian-ambivalent targets
>       virtio-9p: use virtio wrappers to access headers
>       target-ppc: enable virtio endian ambivalent support
>       vhost-net: disable when cross-endian
> 
> Rusty Russell (6):
>       virtio: allow byte swapping for vring
>       virtio-net: use virtio wrappers to access headers
>       virtio-balloon: use virtio wrappers to access page frame numbers
>       virtio-blk: use virtio wrappers to access headers
>       virtio-scsi: use virtio wrappers to access headers
>       virtio-serial-bus: use virtio wrappers to access headers
> 
> 
>  exec.c                            |    8 -
>  hw/9pfs/virtio-9p-device.c        |    3 -
>  hw/block/virtio-blk.c             |   62 ++++++-----
>  hw/char/virtio-serial-bus.c       |   94 ++++++++++------
>  hw/net/vhost_net.c                |   19 +++
>  hw/net/virtio-net.c               |   56 +++++++---
>  hw/scsi/virtio-scsi.c             |   40 ++++---
>  hw/virtio/virtio-balloon.c        |   33 +++---
>  hw/virtio/virtio-pci.c            |   11 +-
>  hw/virtio/virtio-rng.c            |   12 +-
>  hw/virtio/virtio.c                |  216 ++++++++++++++++++++++++++++---------
>  include/hw/virtio/virtio-access.h |  170 +++++++++++++++++++++++++++++
>  include/hw/virtio/virtio.h        |   17 +++
>  include/qom/cpu.h                 |    1 
>  qom/cpu.c                         |    6 +
>  target-ppc/cpu.h                  |    2 
>  target-ppc/translate_init.c       |   15 +++
>  17 files changed, 583 insertions(+), 182 deletions(-)
>  create mode 100644 include/hw/virtio/virtio-access.h
> 
> --
> Greg