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Revision a821ce59

IDa821ce59338c79bb72dc844dd44ea53701965b2b

Added by Michael S. Tsirkin about 12 years ago

virtio: order index/descriptor reads

virtio has the equivalent of:

if (vq->last_avail_index != vring_avail_idx(vq)) {
        read descriptor head at vq->last_avail_index;
    }

In theory, processor can reorder descriptor head
read to happen speculatively before the index read.
this would trigger the following race:

host descriptor head read <- reads invalid head from ring
        guest writes valid descriptor head
        guest writes avail index
    host avail index read <- observes valid index

as a result host will use an invalid head value.
This was not observed in the field by me but after
the experience with the previous two races
I think it is prudent to address this theoretical race condition.

Signed-off-by: Michael S. Tsirkin <>

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