Re: [PATCH v1] vdpa/mlx5: Restore the hardware used index after change map

On 2/9/2021 7:53 PM, Jason Wang wrote:

On 2021/2/10 上午10:30, Si-Wei Liu wrote:

quoted

On 2/8/2021 10:37 PM, Jason Wang wrote:

quoted

On 2021/2/9 下午2:12, Eli Cohen wrote:

quoted

On Tue, Feb 09, 2021 at 11:20:14AM +0800, Jason Wang wrote:

quoted

On 2021/2/8 下午6:04, Eli Cohen wrote:

quoted

On Mon, Feb 08, 2021 at 05:04:27PM +0800, Jason Wang wrote:

quoted

On 2021/2/8 下午2:37, Eli Cohen wrote:

quoted

On Mon, Feb 08, 2021 at 12:27:18PM +0800, Jason Wang wrote:

quoted

On 2021/2/6 上午7:07, Si-Wei Liu wrote:

quoted

On 2/3/2021 11:36 PM, Eli Cohen wrote:

quoted

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.

Also, since the driver also fails to reset the available and 
used
indices upon device reset, fix this here to avoid regression 
caused by
the fact that used index may not be zero upon device reset.

Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for 
supported mlx5
devices")
Signed-off-by: Eli Cohen<redacted>
---
v0 -> v1:
Clear indices upon device reset

     drivers/vdpa/mlx5/net/mlx5_vnet.c | 18 ++++++++++++++++++
     1 file changed, 18 insertions(+)

diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c

b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index 88dde3455bfd..b5fe6d2ad22f 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;
     @@ -1535,6 +1540,16 @@ static void 
teardown_virtqueues(struct
mlx5_vdpa_net *ndev)
         }
     }
     +static void clear_virtqueues(struct mlx5_vdpa_net *ndev)
+{
+    int i;
+
+    for (i = ndev->mvdev.max_vqs - 1; i >= 0; i--) {
+        ndev->vqs[i].avail_idx = 0;
+        ndev->vqs[i].used_idx = 0;
+    }
+}
+
     /* TODO: cross-endian support */
     static inline bool mlx5_vdpa_is_little_endian(struct 
mlx5_vdpa_dev
*mvdev)
     {

@@ -1610,6 +1625,7 @@ 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;

@@ -1654,6 +1670,7 @@ 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;

@@ -1768,6 +1785,7 @@ static void mlx5_vdpa_set_status(struct

vdpa_device *vdev, u8 status)
         if (!status) {
             mlx5_vdpa_info(mvdev, "performing device 
reset\n");
             teardown_driver(ndev);
+        clear_virtqueues(ndev);

The clearing looks fine at the first glance, as it aligns 
with the other
state cleanups floating around at the same place. However, 
the thing is
get_vq_state() is supposed to be called right after to get 
sync'ed with
the latest internal avail_index from device while vq is 
stopped. The
index was saved in the driver software at vq suspension, but 
before the
virtq object is destroyed. We shouldn't clear the avail_index 
too early.

Good point.

There's a limitation on the virtio spec and vDPA framework 
that we can not
simply differ device suspending from device reset.

Are you talking about live migration where you reset the device 
but
still want to know how far it progressed in order to continue 
from the
same place in the new VM?

Yes. So if we want to support live migration at we need:

in src node:
1) suspend the device
2) get last_avail_idx via get_vq_state()

in the dst node:
3) set last_avail_idx via set_vq_state()
4) resume the device

So you can see, step 2 requires the device/driver not to forget the
last_avail_idx.

Just to be sure, what really matters here is the used index. 
Becuase the
vriqtueue itself is copied from the src VM to the dest VM. The 
available
index is alreay there and we know the hardware reads it from there.

So for "last_avail_idx" I meant the hardware internal avail index. 
It's not
stored in the virtqueue so we must migrate it from src to dest and 
set them
through set_vq_state(). Then in the destination, the virtqueue can be
restarted from that index.

Consider this case: driver posted buffers till avail index becomes the
value 50. Hardware is executing but made it till 20 when virtqueue was
suspended due to live migration - this is indicated by hardware used
index equal 20.

So in this case the used index in the virtqueue should be 20? 
Otherwise we need not sync used index itself but all the used 
entries that is not committed to the used ring.

In other word, for mlx5 vdpa there's no such internal last_avail_idx 
stuff maintained by the hardware, right? 

For each device it should have one otherwise it won't work correctly 
during stop/resume. See the codes mlx5_vdpa_get_vq_state() which calls 
query_virtqueue() that build commands to query "last_avail_idx" from 
the hardware:

    MLX5_SET(general_obj_in_cmd_hdr, cmd_hdr, opcode, 
MLX5_CMD_OP_QUERY_GENERAL_OBJECT);
    MLX5_SET(general_obj_in_cmd_hdr, cmd_hdr, obj_type, 
MLX5_OBJ_TYPE_VIRTIO_NET_Q);
    MLX5_SET(general_obj_in_cmd_hdr, cmd_hdr, obj_id, mvq->virtq_id);
    MLX5_SET(general_obj_in_cmd_hdr, cmd_hdr, uid, ndev->mvdev.res.uid);
    err = mlx5_cmd_exec(ndev->mvdev.mdev, in, sizeof(in), out, outlen);
    if (err)
        goto err_cmd;

    obj_context = MLX5_ADDR_OF(query_virtio_net_q_out, out, obj_context);
    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);

Eli should be able to correct me, but this hw_available_index might just 
be a cached value of virtqueue avail_index in the memory from the most 
recent sync. I doubt it's the one you talked about in software 
implementation. If I understand Eli correctly, hardware will always 
reload the latest avail_index from memory whenever it's being sync'ed 
again.

<quote>
The hardware always goes to read the available index from memory. The 
requirement to configure it when creating a new object is still a 
requirement defined by the interface so I must not violate interface 
requirments.
</quote>

If the hardware does everything perfectly that is able to flush pending 
requests, update descriptors, rings plus used indices all at once before 
the suspension, there's no need for hardware to maintain a separate 
internal index than the h/w used_index. The hardware can get started 
from the saved used_index upon resuming. I view this is of (hardware) 
implementation choices and thought it does not violate the virtio spec?

quoted

And the used_idx in the virtqueue is always in sync with the hardware 
used_index, and hardware is supposed to commit pending used buffers 
to the ring while bumping up the hardware used_index (and also 
committed to memory) altogether prior to suspension, is my 
understanding correct here? Double checking if this is the expected 
semantics of what 
modify_virtqueue(MLX5_VIRTIO_NET_Q_OBJECT_STATE_SUSPEND) should achieve.

If the above is true, then it looks to me for mlx5 vdpa we should 
really return h/w used_idx rather than the last_avail_idx through 
get_vq_state(), in order to reconstruct the virt queue state post 
live migration. For the set_map case, the internal last_avail_idx 
really doesn't matter, although both indices are saved and restored 
transparently as-is.

Right, a subtle thing here is that: for the device that might have 
can't not complete all virtqueue requests during vq suspending, the 
"last_avail_idx" might not be equal to the hardware used_idx. Thing 
might be true for the storage devices that needs to connect to a 
remote backend. But this is not the case of networking device, so 
last_avail_idx should be equal to hardware used_idx here. 

Eli, since it's your hardware, does it work this way? i.e. does the 
firmware interface see a case where virtqueue requests can't be 
completed before suspending vq?

But using the "last_avail_idx" or hardware avail_idx should always be 
better in this case since it's guaranteed to correct and will have 
less confusion. We use this convention in other types of vhost 
backends (vhost-kernel, vhost-user).

So looking at mlx5_set_vq_state(), it probably won't work since it 
doesn't not set either hardware avail_idx or hardware used_idx:

The saved mvq->avail_idx will be used to recreate hardware virtq object 
and the used index in create_virtqueue(), once status DRIVER_OK is set. 
I suspect we should pass the index to mvq->used_idx in 
mlx5_vdpa_set_vq_state() below instead.


Thanks,
-Siwei

static int mlx5_vdpa_set_vq_state(struct vdpa_device *vdev, u16 idx,
                  const struct vdpa_vq_state *state)
{
    struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
    struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
    struct mlx5_vdpa_virtqueue *mvq = &ndev->vqs[idx];

    if (mvq->fw_state == MLX5_VIRTIO_NET_Q_OBJECT_STATE_RDY) {
        mlx5_vdpa_warn(mvdev, "can't modify available index\n");
        return -EINVAL;
    }

    mvq->avail_idx = state->avail_index;
    return 0;
}

Depends on the hardware, we should either set hardware used_idx or 
hardware avail_idx here.

I think we need to clarify how device is supposed to work in the 
virtio spec.

Thanks

quoted

-Siwei

quoted

quoted

Now the vritqueue is copied to the new VM and the
hardware now has to continue execution from index 20. We need to tell
the hardware via configuring the last used_index.

If the hardware can not sync the index from the virtqueue, the 
driver can do the synchronization by make the last_used_idx equals 
to used index in the virtqueue.

Thanks

quoted

  So why don't we
restore the used index?

quoted

quoted

So it puzzles me why is set_vq_state() we do not communicate the 
saved
used index.

We don't do that since:

1) if the hardware can sync its internal used index from the 
virtqueue
during device, then we don't need it
2) if the hardware can not sync its internal used index, the 
driver (e.g as
you did here) can do that.

But there's no way for the hardware to deduce the internal avail 
index from
the virtqueue, that's why avail index is sycned.

Thanks