Re: [PATCH 2/2] vdpa/mlx5: Restore the hardware used index after change map

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On 2021/2/1 下午2:38, Eli Cohen wrote:
On Mon, Feb 01, 2021 at 02:00:35PM +0800, Jason Wang wrote:
On 2021/2/1 下午1:52, Eli Cohen wrote:
On Mon, Feb 01, 2021 at 11:36:23AM +0800, Jason Wang wrote:
On 2021/2/1 上午2:55, Eli Cohen wrote:
On Fri, Jan 29, 2021 at 11:49:45AM +0800, Jason Wang wrote:
On 2021/1/28 下午9:41, Eli Cohen wrote:
When a change of memory map occurs, the hardware resources are destroyed
and then re-created again with the new memory map. In such case, we need
to restore the hardware available and used indices. The driver failed to
restore the used index which is added here.

Fixes 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Eli Cohen <elic@xxxxxxxxxx>
A question. Does this mean after a vq is suspended, the hw used index is not
equal to vq used index?
Surely there is just one "Used index" for a VQ. What I was trying to say
is that after the VQ is suspended, I read the used index by querying the
hardware. The read result is the used index that the hardware wrote to
memory.
Just to make sure I understand here. So it looks to me we had two index. The
first is the used index which is stored in the memory/virtqueue, the second
is the one that is stored by the device.

It is the structures defined in the virtio spec in 2.6.6 for the
available ring and 2.6.8 for the used ring. As you know these the
available ring is written to by the driver and read by the device. The
opposite happens for the used index.

Right, so for used index it was wrote by device. And the device should have
an internal used index value that is used to write to the used ring. And the
code is used to sync the device internal used index if I understand this
correctly.


The reason I need to restore the last known indices is for the new
hardware objects to sync on the last state and take over from there.

Right, after the vq suspending, the questions are:

1) is hardware internal used index might not be the same with the used index
in the virtqueue?

Generally the answer is no because the hardware is the only one writing
it. New objects start up with the initial value configured to them upon
creation. This value was zero before this change.
You could argue that since the hardware has access to virtqueue memory,
it could just read the value from there but it does not.


I see.



or

2) can we simply sync the virtqueue's used index to the hardware's used
index?

Theoretically it could be done but that's not how the hardware works.
One reason is that is not supposed to read from that area. But it is
really hardware implementation detail.


I get you now.

So

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

Thanks


    After the I create the new hardware object, I need to tell it
what is the used index (and the available index) as a way to sync it
with the existing VQ.
For avail index I understand that the hardware index is not synced with the
avail index stored in the memory/virtqueue. The question is used index, if
the hardware one is not synced with the one in the virtqueue. It means after
vq is suspended,  some requests is not completed by the hardware (e.g the
buffer were not put to used ring).

This may have implications to live migration, it means those unaccomplished
requests needs to be migrated to the destination and resubmitted to the
device. This looks not easy.

Thanks


This sync is especially important when a change of map occurs while the
VQ was already used (hence the indices are likely to be non zero). This
can be triggered by hot adding memory after the VQs have been used.

Thanks


---
     drivers/vdpa/mlx5/net/mlx5_vnet.c | 7 +++++++
     1 file changed, 7 insertions(+)

diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index 549ded074ff3..3fc8588cecae 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
@@ -87,6 +87,7 @@ struct mlx5_vq_restore_info {
     	u64 device_addr;
     	u64 driver_addr;
     	u16 avail_index;
+	u16 used_index;
     	bool ready;
     	struct vdpa_callback cb;
     	bool restore;
@@ -121,6 +122,7 @@ struct mlx5_vdpa_virtqueue {
     	u32 virtq_id;
     	struct mlx5_vdpa_net *ndev;
     	u16 avail_idx;
+	u16 used_idx;
     	int fw_state;
     	/* keep last in the struct */
@@ -804,6 +806,7 @@ static int create_virtqueue(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtque
     	obj_context = MLX5_ADDR_OF(create_virtio_net_q_in, in, obj_context);
     	MLX5_SET(virtio_net_q_object, obj_context, hw_available_index, mvq->avail_idx);
+	MLX5_SET(virtio_net_q_object, obj_context, hw_used_index, mvq->used_idx);
     	MLX5_SET(virtio_net_q_object, obj_context, queue_feature_bit_mask_12_3,
     		 get_features_12_3(ndev->mvdev.actual_features));
     	vq_ctx = MLX5_ADDR_OF(virtio_net_q_object, obj_context, virtio_q_context);
@@ -1022,6 +1025,7 @@ static int connect_qps(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *m
     struct mlx5_virtq_attr {
     	u8 state;
     	u16 available_index;
+	u16 used_index;
     };
     static int query_virtqueue(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *mvq,
@@ -1052,6 +1056,7 @@ static int query_virtqueue(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueu
     	memset(attr, 0, sizeof(*attr));
     	attr->state = MLX5_GET(virtio_net_q_object, obj_context, state);
     	attr->available_index = MLX5_GET(virtio_net_q_object, obj_context, hw_available_index);
+	attr->used_index = MLX5_GET(virtio_net_q_object, obj_context, hw_used_index);
     	kfree(out);
     	return 0;
@@ -1602,6 +1607,7 @@ static int save_channel_info(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqu
     		return err;
     	ri->avail_index = attr.available_index;
+	ri->used_index = attr.used_index;
     	ri->ready = mvq->ready;
     	ri->num_ent = mvq->num_ent;
     	ri->desc_addr = mvq->desc_addr;
@@ -1646,6 +1652,7 @@ static void restore_channels_info(struct mlx5_vdpa_net *ndev)
     			continue;
     		mvq->avail_idx = ri->avail_index;
+		mvq->used_idx = ri->used_index;
     		mvq->ready = ri->ready;
     		mvq->num_ent = ri->num_ent;
     		mvq->desc_addr = ri->desc_addr;

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