Virtualization - Feb 2021 - [PATCH 2/3] mlx5_vdpa: fix feature negotiation across device reset

When feature VIRTIO_NET_F_MTU is negotiated on mlx5_vdpa,
22 extra bytes worth of MTU length is shown in guest.
This is because the mlx5_query_port_max_mtu API returns
the "hardware" MTU value, which does not just contain the
Ethernet payload, but includes extra lengths starting
from the Ethernet header up to the FCS altogether.

Fix the MTU so packets won't get dropped silently.

Signed-off-by: Si-Wei Liu <si-wei.liu at oracle.com>
---
 drivers/vdpa/mlx5/core/mlx5_vdpa.h |  4 ++++
 drivers/vdpa/mlx5/net/mlx5_vnet.c  | 15 ++++++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/vdpa/mlx5/core/mlx5_vdpa.h
b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
index 08f742f..b6cc53b 100644
--- a/drivers/vdpa/mlx5/core/mlx5_vdpa.h
+++ b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
@@ -4,9 +4,13 @@
 #ifndef __MLX5_VDPA_H__
 #define __MLX5_VDPA_H__
 
+#include <linux/etherdevice.h>
+#include <linux/if_vlan.h>
 #include <linux/vdpa.h>
 #include <linux/mlx5/driver.h>
 
+#define MLX5V_ETH_HARD_MTU (ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN)
+
 struct mlx5_vdpa_direct_mr {
 	u64 start;
 	u64 end;
diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index dc88559..b8416c4 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
@@ -1907,6 +1907,19 @@ static int mlx5_get_vq_irq(struct vdpa_device *vdv, u16
idx)
 	.free = mlx5_vdpa_free,
 };
 
+static int query_mtu(struct mlx5_core_dev *mdev, u16 *mtu)
+{
+	u16 hw_mtu;
+	int err;
+
+	err = mlx5_query_nic_vport_mtu(mdev, &hw_mtu);
+	if (err)
+		return err;
+
+	*mtu = hw_mtu - MLX5V_ETH_HARD_MTU;
+	return 0;
+}
+
 static int alloc_resources(struct mlx5_vdpa_net *ndev)
 {
 	struct mlx5_vdpa_net_resources *res = &ndev->res;
@@ -1992,7 +2005,7 @@ static int mlx5v_probe(struct auxiliary_device *adev,
 	init_mvqs(ndev);
 	mutex_init(&ndev->reslock);
 	config = &ndev->config;
-	err = mlx5_query_nic_vport_mtu(mdev, &ndev->mtu);
+	err = query_mtu(mdev, &ndev->mtu);
 	if (err)
 		goto err_mtu;
 
-- 
1.8.3.1

The mlx_features denotes the capability for which
set of virtio features is supported by device. In
principle, this field needs not be cleared during
virtio device reset, as this capability is static
and does not change across reset.

In fact, the current code may have the assumption
that mlx_features can be reloaded from firmware
via the .get_features ops after device is reset
(via the .set_status ops), which is unfortunately
not true. The userspace VMM might save a copy
of backend capable features and won't call into
kernel again to get it on reset. This causes all
virtio features getting disabled on newly created
virtqs after device reset, while guest would hold
mismatched view of available features. For e.g.,
the guest may still assume tx checksum offload
is available after reset and feature negotiation,
causing frames with bogus (incomplete) checksum
transmitted on the wire.

Signed-off-by: Si-Wei Liu <si-wei.liu at oracle.com>
---
 drivers/vdpa/mlx5/net/mlx5_vnet.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index b8416c4..aa6f8cd 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
@@ -1788,7 +1788,6 @@ static void mlx5_vdpa_set_status(struct vdpa_device *vdev,
u8 status)
 		clear_virtqueues(ndev);
 		mlx5_vdpa_destroy_mr(&ndev->mvdev);
 		ndev->mvdev.status = 0;
-		ndev->mvdev.mlx_features = 0;
 		++mvdev->generation;
 		return;
 	}
-- 
1.8.3.1

While virtq is stopped,  get_vq_state() is supposed to
be  called to  get  sync'ed  with  the latest internal
avail_index from device. The saved avail_index is used
to restate  the virtq  once device is started.  Commit
b35ccebe3ef7 introduced the clear_virtqueues() routine
to  reset  the saved  avail_index,  however, the index
gets cleared a bit earlier before get_vq_state() tries
to read it. This would cause consistency problems when
virtq is restarted, e.g. through a series of link down
and link up events. We  could  defer  the  clearing of
avail_index  to  until  the  device  is to be started,
i.e. until  VIRTIO_CONFIG_S_DRIVER_OK  is set again in
set_status().

Fixes: b35ccebe3ef7 ("vdpa/mlx5: Restore the hardware used index after
change map")
Signed-off-by: Si-Wei Liu <si-wei.liu at oracle.com>
---
 drivers/vdpa/mlx5/net/mlx5_vnet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index aa6f8cd..444ab58 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
@@ -1785,7 +1785,6 @@ 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);
 		mlx5_vdpa_destroy_mr(&ndev->mvdev);
 		ndev->mvdev.status = 0;
 		++mvdev->generation;
@@ -1794,6 +1793,7 @@ static void mlx5_vdpa_set_status(struct vdpa_device *vdev,
u8 status)
 
 	if ((status ^ ndev->mvdev.status) & VIRTIO_CONFIG_S_DRIVER_OK) {
 		if (status & VIRTIO_CONFIG_S_DRIVER_OK) {
+			clear_virtqueues(ndev);
 			err = setup_driver(ndev);
 			if (err) {
 				mlx5_vdpa_warn(mvdev, "failed to setup driver\n");
-- 
1.8.3.1

On 2021/2/6 ??8:29, Si-Wei Liu wrote:
> When feature VIRTIO_NET_F_MTU is negotiated on mlx5_vdpa,
> 22 extra bytes worth of MTU length is shown in guest.
> This is because the mlx5_query_port_max_mtu API returns
> the "hardware" MTU value, which does not just contain the
> Ethernet payload, but includes extra lengths starting
> from the Ethernet header up to the FCS altogether.
>
> Fix the MTU so packets won't get dropped silently.
>
> Signed-off-by: Si-Wei Liu <si-wei.liu at oracle.com>

Acked-by: Jason Wang <jasowang at redhat.com>

> ---
>   drivers/vdpa/mlx5/core/mlx5_vdpa.h |  4 ++++
>   drivers/vdpa/mlx5/net/mlx5_vnet.c  | 15 ++++++++++++++-
>   2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vdpa/mlx5/core/mlx5_vdpa.h
b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> index 08f742f..b6cc53b 100644
> --- a/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> +++ b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> @@ -4,9 +4,13 @@
>   #ifndef __MLX5_VDPA_H__
>   #define __MLX5_VDPA_H__
>   
> +#include <linux/etherdevice.h>
> +#include <linux/if_vlan.h>
>   #include <linux/vdpa.h>
>   #include <linux/mlx5/driver.h>
>   
> +#define MLX5V_ETH_HARD_MTU (ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN)
> +
>   struct mlx5_vdpa_direct_mr {
>   	u64 start;
>   	u64 end;
> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> index dc88559..b8416c4 100644
> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> @@ -1907,6 +1907,19 @@ static int mlx5_get_vq_irq(struct vdpa_device *vdv,
u16 idx)
>   	.free = mlx5_vdpa_free,
>   };
>   
> +static int query_mtu(struct mlx5_core_dev *mdev, u16 *mtu)
> +{
> +	u16 hw_mtu;
> +	int err;
> +
> +	err = mlx5_query_nic_vport_mtu(mdev, &hw_mtu);
> +	if (err)
> +		return err;
> +
> +	*mtu = hw_mtu - MLX5V_ETH_HARD_MTU;
> +	return 0;
> +}
> +
>   static int alloc_resources(struct mlx5_vdpa_net *ndev)
>   {
>   	struct mlx5_vdpa_net_resources *res = &ndev->res;
> @@ -1992,7 +2005,7 @@ static int mlx5v_probe(struct auxiliary_device *adev,
>   	init_mvqs(ndev);
>   	mutex_init(&ndev->reslock);
>   	config = &ndev->config;
> -	err = mlx5_query_nic_vport_mtu(mdev, &ndev->mtu);
> +	err = query_mtu(mdev, &ndev->mtu);
>   	if (err)
>   		goto err_mtu;
>