Re: [Qemu-devel] [PATCH v8 11/20] virtio: add endian-ambivalent support to VirtIODevice

[Alexander Graf]

On 13.06.14 14:14, Greg Kurz wrote:
> On Fri, 13 Jun 2014 13:46:39 +0200
> Alexander Graf <address@hidden> wrote:
>
> > On 13.06.14 13:23, Greg Kurz wrote:
> > > Some CPU families can dynamically change their endianness. This means we
> > > can have little endian ppc or big endian arm guests for example. This has
> > > an impact on legacy virtio data structures since they are target endian.
> > > We hence introduce a new property to track the endianness of each virtio
> > > device. It is reasonnably assumed that endianness won't change while the
> > > device is in use : we hence capture the device endianness when it gets
> > > reset.
> > >
> > > We migrate this property in a subsection, after the device descriptor. This
> > > means the load code must not rely on it until it is restored. As a consequence,
> > > the vring sanity checks had to be moved after the call to vmstate_load_state().
> > > We enforce paranoia by poisoning the property at the begining of virtio_load().
> > >
> > > Signed-off-by: Greg Kurz <address@hidden>
> > > ---
> > >
> > >    Changes since last version (virtio migration RFC):
> > >    - endianness cached at device reset time
> > >    - reworked migration support
> > >
> > >    hw/virtio/virtio-pci.c     |    8 ++--
> > >    hw/virtio/virtio.c         |  100 
> > > +++++++++++++++++++++++++++++++++++++++-----
> > >    include/hw/virtio/virtio.h |   12 ++++-
> > >    3 files changed, 102 insertions(+), 18 deletions(-)
> > >
> > > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > > index 390c8d2..2ffceb8 100644
> > > --- a/hw/virtio/virtio-pci.c
> > > +++ b/hw/virtio/virtio-pci.c
> > > @@ -406,13 +406,13 @@ static uint64_t virtio_pci_config_read(void *opaque, 
> > > hwaddr addr,
> > >            break;
> > >        case 2:
> > >            val = virtio_config_readw(vdev, addr);
> > > -        if (virtio_is_big_endian()) {
> > > +        if (virtio_is_big_endian(vdev)) {
> > >                val = bswap16(val);
> > >            }
> > >            break;
> > >        case 4:
> > >            val = virtio_config_readl(vdev, addr);
> > > -        if (virtio_is_big_endian()) {
> > > +        if (virtio_is_big_endian(vdev)) {
> > >                val = bswap32(val);
> > >            }
> > >            break;
> > > @@ -440,13 +440,13 @@ static void virtio_pci_config_write(void *opaque, hwaddr 
> > > addr,
> > >            virtio_config_writeb(vdev, addr, val);
> > >            break;
> > >        case 2:
> > > -        if (virtio_is_big_endian()) {
> > > +        if (virtio_is_big_endian(vdev)) {
> > >                val = bswap16(val);
> > >            }
> > >            virtio_config_writew(vdev, addr, val);
> > >            break;
> > >        case 4:
> > > -        if (virtio_is_big_endian()) {
> > > +        if (virtio_is_big_endian(vdev)) {
> > >                val = bswap32(val);
> > >            }
> > >            virtio_config_writel(vdev, addr, val);
> > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > > index 16b73d9..6235df8 100644
> > > --- a/hw/virtio/virtio.c
> > > +++ b/hw/virtio/virtio.c
> > > @@ -545,6 +545,24 @@ void virtio_set_status(VirtIODevice *vdev, uint8_t val)
> > >        vdev->status = val;
> > >    }
> > >    
> > > +static void virtio_set_endian_target_default(VirtIODevice *vdev)
> > > +{
> > > +#ifdef TARGET_WORDS_BIGENDIAN
> > > +    vdev->device_endian = VIRTIO_DEVICE_ENDIAN_BIG;
> > > +#else
> > > +    vdev->device_endian = VIRTIO_DEVICE_ENDIAN_LITTLE;
> > > +#endif
> > > +}
> > > +
> > > +static void virtio_set_endian_cpu(VirtIODevice *vdev, CPUState *cpu)
> > > +{
> > > +    if (cpu_virtio_is_big_endian(cpu)) {
> > > +        vdev->device_endian = VIRTIO_DEVICE_ENDIAN_BIG;
> > > +    } else {
> > > +        vdev->device_endian = VIRTIO_DEVICE_ENDIAN_LITTLE;
> > > +    }
> > > +}
> > > +
> > >    void virtio_reset(void *opaque)
> > >    {
> > >        VirtIODevice *vdev = opaque;
> > > @@ -552,6 +570,13 @@ void virtio_reset(void *opaque)
> > >        int i;
> > >    
> > >        virtio_set_status(vdev, 0);
> > > +    if (current_cpu) {
> > > +        /* Guest initiated reset */
> > > +        virtio_set_endian_cpu(vdev, current_cpu);
> > > +    } else {
> > > +        /* System reset */
> > > +        virtio_set_endian_target_default(vdev);
> > > +    }
> > >    
> > >        if (k->reset) {
> > >            k->reset(vdev);
> > > @@ -840,6 +865,28 @@ void virtio_notify_config(VirtIODevice *vdev)
> > >        virtio_notify_vector(vdev, vdev->config_vector);
> > >    }
> > >    
> > > +static bool virtio_device_endian_needed(void *opaque)
> > > +{
> > > +    VirtIODevice *vdev = opaque;
> > > +
> > > +    assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
> > > +#ifdef TARGET_WORDS_BIGENDIAN
> > > +    return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_LITTLE;
> > > +#else
> > > +    return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
> > > +#endif
> > if (target_words_bigendian()) {
> >       return virtio_is_big_endian();
> > } else {
> >       return !virtio_is_big_endian();
> > }
> >
> > Then we have one less dependency on TARGET_WORDS_BIGENDIAN which means a
> > step towards compiling virtio once ;).
> >
> >
> > Alex
> Heh ! It used to be target_words_bigendian() in the first place and I turned
> it into TARGET_WORDS_BIGENDIAN to avoid "hey virtio isn't soon to be common
> why don't you sort this out at compile time?" remarks. :)
>
> Since we are definitely not on a fastpath here, I'll gladly fix this.

Yeah, just combine it with virtio_set_endian_target_default.

static enum foo virtio_default_endian(vdev)
{
    if (target_words_bigendian()) {
        return VIRTIO_DEVICE_ENDIAN_BIG;
    ....
}

Instead of calling virtio_set_endian_target_default, you do

    vdev->device_endian = virtio_default_endian(vdev);

and

static bool virtio_biendian_section_needed(vdev)
{
    assert(unknown);
    return vdev->device_endian != virtio_default_endian(vdev);
}


Alex