On 11/5/20 2:06 PM, Cornelia Huck wrote:
> On Thu, 5 Nov 2020 13:43:51 +0100
> Philippe Mathieu-Daudé <phi...@redhat.com> wrote:
>
>> Per [1] (Terminology):
>>
>> Legacy interfaces are not required; ie. don’t implement them
>> unless you have a need for backwards compatibility!
>>
>> [2] (Version 1.0):
>>
>> The device configuration space uses the little-endian format
>> for multi-byte fields.
>>
>> and [3] (Legacy Interface):
>>
>> for legacy interfaces, device configuration space is generally
>> the guest’s native endian, rather than PCI’s little-endian.
>> The correct endian-ness is documented for each device.
>>
>> Add the --disable-virtio-legacy configure flag to produce builds
>> with VIRTIO 1.0 only, and the --enable-virtio-legacy to include
>> legacy VIRTIO support (supporting legacy VIRTIO is the default).
>
> This is only dealing with endianess issues; there are other differences
> on the control plane as well.
>
> Currently, virtio-pci has the option to make devices non-transitional,
> but virtio-ccw has not (only for device types). For virtio-mmio, you
> need to select one of legacy or non-transitional, IIRC.
>
>>
>> [1]
>> http://docs.oasis-open.org/virtio/virtio/v1.0/cs04/virtio-v1.0-cs04.html#x1-60001
>> [2]
>> http://docs.oasis-open.org/virtio/virtio/v1.0/cs04/virtio-v1.0-cs04.html#x1-170003
>> [3]
>> http://docs.oasis-open.org/virtio/virtio/v1.0/cs04/virtio-v1.0-cs04.html#x1-200003
>>
>> Signed-off-by: Philippe Mathieu-Daudé <phi...@redhat.com>
>> ---
>> configure | 10 ++++++++++
>> meson.build | 1 +
>> include/hw/virtio/virtio-access.h | 19 +++++--------------
>> hw/virtio/virtio-legacy.c | 29 +++++++++++++++++++++++++++++
>> hw/virtio/meson.build | 1 +
>> 5 files changed, 46 insertions(+), 14 deletions(-)
>> create mode 100644 hw/virtio/virtio-legacy.c
>
> (...)
>
>> diff --git a/include/hw/virtio/virtio-access.h
>> b/include/hw/virtio/virtio-access.h
>> index 6818a23a2d3..b6c060f8cc6 100644
>> --- a/include/hw/virtio/virtio-access.h
>> +++ b/include/hw/virtio/virtio-access.h
>> @@ -20,24 +20,15 @@
>> #include "hw/virtio/virtio.h"
>> #include "hw/virtio/virtio-bus.h"
>>
>> -#if defined(TARGET_PPC64) || defined(TARGET_ARM)
>> -#define LEGACY_VIRTIO_IS_BIENDIAN 1
>> -#endif
>> -
>> +#ifdef CONFIG_VIRTIO_LEGACY
>> +bool virtio_access_is_big_endian(VirtIODevice *vdev);
>> +#else
>> static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
>> {
>> -#if defined(LEGACY_VIRTIO_IS_BIENDIAN)
>> - return virtio_is_big_endian(vdev);
>> -#elif defined(TARGET_WORDS_BIGENDIAN)
>> - if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
>> - /* Devices conforming to VIRTIO 1.0 or later are always LE. */
>> - return false;
>> - }
>> - return true;
>> -#else
>> + /* Devices conforming to VIRTIO 1.0 or later are always LE. */
>> return false;
>
> This will make migration from a QEMU that has devices for which 1.0 has
> not been negotiated fail.
Oh good point... Not as easy as I thought then :/
Now I'm seeing plenty of possible problems.
Commits 9b3a35ec823 & d55f518248f help a bit:
("virtio: verify that legacy support is not accidentally on")
("virtio: skip legacy support check on machine types less than 5.1")
Thanks for warning :)
>
>> -#endif
>> }
>> +#endif
>>
>> static inline uint16_t virtio_lduw_phys(VirtIODevice *vdev, hwaddr pa)
>> {
>
