git: virtio: Fix cpu_lfence() wrong placement in virtqueue_dequeue()

[Aaron LI]

commit 8dd5dbc6b251e624a1739962ec7b7ba6d5a30814
Author: Aaron LI <aly at aaronly.me>
Date:   Mon Sep 29 21:28:30 2025 +0800

    virtio: Fix cpu_lfence() wrong placement in virtqueue_dequeue()
    
    The cpu_lfence() should be placed just after reading the 'used->idx',
    before the 'vq_used_cons_idx' equality check and 'used->ring[]' access.
    This is the correct use pattern of lfence.
    
    More importantly, this fix an infinite loop in virtqueue_poll() that
    happened when GCC 12.5 inlined the whole virtqueue_dequeue().  GCC
    thought 'vq->vq_ring.used->idx' wouldn't change during the loop, so
    hoisted the 'if (vq->vq_used_cons_idx == vq->vq_ring.used->idx)' into
    its own loop and thus an infinite loop.
    
    Maybe it's better to use atomic ops to read the 'vq_ring.used->idx' and
    'vq_ring.used->flags' fields, which are updated by the host side.  Need
    to investigate this later.

Summary of changes:
 sys/dev/virtual/virtio/virtio/virtqueue.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/8dd5dbc6b251e624a1739962ec7b7ba6d5a30814


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