Hi, I had a few more queries here as well.
On 3/24/25 7:29 PM, Sahil Siddiq wrote:
Detect when used descriptors are ready for consumption by the guest via packed virtqueues and forward them from the device to the guest. Signed-off-by: Sahil Siddiq <sahil...@proton.me> --- Changes from v4 -> v5: - New commit. - vhost-shadow-virtqueue.c: (vhost_svq_more_used): Split into vhost_svq_more_used_split and vhost_svq_more_used_packed. (vhost_svq_enable_notification): Handle split and packed vqs. (vhost_svq_disable_notification): Likewise. (vhost_svq_get_buf): Split into vhost_svq_get_buf_split and vhost_svq_get_buf_packed. (vhost_svq_poll): Use new functions. hw/virtio/vhost-shadow-virtqueue.c | 121 ++++++++++++++++++++++++++--- 1 file changed, 110 insertions(+), 11 deletions(-) diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c index 126957231d..8430b3c94a 100644 --- a/hw/virtio/vhost-shadow-virtqueue.c +++ b/hw/virtio/vhost-shadow-virtqueue.c @@ -463,7 +463,7 @@ static void vhost_handle_guest_kick_notifier(EventNotifier *n) vhost_handle_guest_kick(svq); }-static bool vhost_svq_more_used(VhostShadowVirtqueue *svq)+static bool vhost_svq_more_used_split(VhostShadowVirtqueue *svq) { uint16_t *used_idx = &svq->vring.used->idx; if (svq->last_used_idx != svq->shadow_used_idx) { @@ -475,6 +475,22 @@ static bool vhost_svq_more_used(VhostShadowVirtqueue *svq) return svq->last_used_idx != svq->shadow_used_idx; }+static bool vhost_svq_more_used_packed(VhostShadowVirtqueue *svq)+{ + bool avail_flag, used_flag, used_wrap_counter; + uint16_t last_used_idx, last_used, flags; + + last_used_idx = svq->last_used_idx; + last_used = last_used_idx & ~(1 << VRING_PACKED_EVENT_F_WRAP_CTR);
In the linux kernel, last_used is calculated as: last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR)) ...instead of... last_used_idx & ~(1 << VRING_PACKED_EVENT_F_WRAP_CTR) Isn't the second option good enough if last_used_idx is uint16_t and VRING_PACKED_EVENT_F_WRAP_CTR is defined as 15.
+ used_wrap_counter = !!(last_used_idx & (1 << VRING_PACKED_EVENT_F_WRAP_CTR)); + + flags = le16_to_cpu(svq->vring_packed.vring.desc[last_used].flags); + avail_flag = !!(flags & (1 << VRING_PACKED_DESC_F_AVAIL)); + used_flag = !!(flags & (1 << VRING_PACKED_DESC_F_USED)); + + return avail_flag == used_flag && used_flag == used_wrap_counter; +} +
Also in the implementation of vhost_svq_more_used_split() [1], I haven't understood why the following condition: svq->last_used_idx != svq->shadow_used_idx is checked before updating the value of "svq->shadow_used_idx": svq->shadow_used_idx = le16_to_cpu(*(volatile uint16_t *)used_idx) Thanks, Sahil [1] https://gitlab.com/qemu-project/qemu/-/blob/master/hw/virtio/vhost-shadow-virtqueue.c#L387
