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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Thu, 15 Dec 2016 14:37:08 +0100
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Subject: Re: [Qemu-devel] commit virtio: recalculate vq->inuse after
 migration might cause last_avail_idx vs. used_idx failure
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To: Halil Pasic <pasic@linux.vnet.ibm.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>, QEMU Developers <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>



On 15/12/2016 12:32, Halil Pasic wrote:
> static inline uint16_t vring_avail_idx(VirtQueue *vq)
> {
>     hwaddr pa;
>     pa = vq->vring.avail + offsetof(VRingAvail, idx);
>     vq->shadow_avail_idx = virtio_lduw_phys(vq->vdev, pa);
> 
> we should have an endiannes handling here before assigning shadow_avail_idx I guess
> 
>     return vq->shadow_avail_idx;
> } 

Endianness is already handled:

static inline uint16_t virtio_lduw_phys(VirtIODevice *vdev, hwaddr pa)
{
    if (virtio_access_is_big_endian(vdev)) {
        return lduw_be_phys(&address_space_memory, pa);
    }
    return lduw_le_phys(&address_space_memory, pa);
}

> I will meditate a bit more on this and probably create a patch to fix it.
> 
> What make me wonder is that according to the reports live migration usually
> works (ca 1% fails)...

What is the backtrace of the vring_avail_idx call?  If your device is
virtio 1.0, and vdev->guest_features has not been initialized correctly,
you might incorrectly treat LE virtio 1.0 data as BE virtio 0.9 data:

    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;

Thanks,

Paolo