linux-kernel - Re: [PATCH v3 1/2] vdpa/mlx5: Set speed and duplex of vDPA devices to UNKNOWN

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Message-ID: <bb010d16-a083-44da-8a72-3ada4c6f8056@digitalocean.com>
Date: Fri, 8 Nov 2024 17:44:44 -0600
From: Carlos Bilbao <cbilbao@...italocean.com>
To: "Michael S. Tsirkin" <mst@...hat.com>,
 Dragos Tatulea <dtatulea@...dia.com>
Cc: Carlos Bilbao <carlos.bilbao.osdev@...il.com>, jasowang@...hat.com,
 shannon.nelson@....com, sashal@...nel.org, alvaro.karsz@...id-run.com,
 christophe.jaillet@...adoo.fr, steven.sistare@...cle.com, bilbao@...edu,
 xuanzhuo@...ux.alibaba.com, johnah.palmer@...cle.com, eperezma@...hat.com,
 cratiu@...dia.com, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH v3 1/2] vdpa/mlx5: Set speed and duplex of vDPA devices to
 UNKNOWN

Hello,

On 11/8/24 5:51 AM, Michael S. Tsirkin wrote:
> On Fri, Nov 08, 2024 at 10:31:58AM +0100, Dragos Tatulea wrote:
>>
>> On 07.11.24 22:50, Michael S. Tsirkin wrote:
>>> On Wed, Sep 04, 2024 at 10:11:14AM -0500, Carlos Bilbao wrote:
>>>> From: Carlos Bilbao <cbilbao@...italocean.com>
>>>>
>>>> Initialize the speed and duplex fields in virtio_net_config to UNKNOWN.
>>>> This is needed because mlx5_vdpa vDPA devices currently do not support the
>>>> VIRTIO_NET_F_SPEED_DUPLEX feature which reports speed and duplex.
>>> I see no logic here. Without this feature bit, guests will not read
>>> this field, why do we suddenly need to initialize it?
>>>
>> IIRC, Carlos was reading data via ioctl VHOST_VDPA_GET_CONFIG which calls
>> .get_config() directly, always exposing the speed and duplex config fields [0].
>> Carlos, was this the case?
>>
>> [0] https://lore.kernel.org/lkml/afcbf041-7613-48e6-8088-9d52edd907ff@nvidia.com/T/
>>
>> Thanks,
>> Dragos
> Basically, driver should ignore these if feature is not set.

There _is_ a chance the guest could read this field; As Dragos mentioned, I
was using the VHOST_VDPA_GET_CONFIG ioctl from userspace, and the incorrect
field initialization led me to believe I was in half-duplex mode --
something people told me they hadn't seen since the 80s. But as Andrew
(CCed) mentioned, "If the speed is 0, does duplex even matter?".

I also tried to remove the pointless ioctl call altogether to avoid further
confusion [0] but that wasn't viable due to spec compliance.

Initializing the fields seems the simple solution here (and, IMHO, more
generally, it's as a good programming practice)

Thanks for looking into this!

Best,
Carlos

[0] https://lore.kernel.org/lkml/CACGkMEvfdUYLjx-Z+oB11XW-54ErJsQMKcnu2p=dsj5N_BiEKw@mail.gmail.com/

>
>
>>>> Add
>>>> needed helper cpu_to_mlx5vdpa32() to convert endianness of speed.
>>>>
>>>> Signed-off-by: Carlos Bilbao <cbilbao@...italocean.com>
>>>> Reviewed-by: Dragos Tatulea <dtatulea@...dia.com>
>>>> ---
>>>>  drivers/vdpa/mlx5/net/mlx5_vnet.c | 12 ++++++++++++
>>>>  1 file changed, 12 insertions(+)
>>>>
>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>> index b56aae3f7be3..41ca268d43ff 100644
>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>> @@ -173,6 +173,11 @@ static __virtio16 cpu_to_mlx5vdpa16(struct mlx5_vdpa_dev *mvdev, u16 val)
>>>>  	return __cpu_to_virtio16(mlx5_vdpa_is_little_endian(mvdev), val);
>>>>  }
>>>>  
>>>> +static __virtio32 cpu_to_mlx5vdpa32(struct mlx5_vdpa_dev *mvdev, u32 val)
>>>> +{
>>>> +	return __cpu_to_virtio32(mlx5_vdpa_is_little_endian(mvdev), val);
>>>> +}
>>>> +
>>>>  static u16 ctrl_vq_idx(struct mlx5_vdpa_dev *mvdev)
>>>>  {
>>>>  	if (!(mvdev->actual_features & BIT_ULL(VIRTIO_NET_F_MQ)))
>>>> @@ -3433,6 +3438,13 @@ static int mlx5_vdpa_dev_add(struct vdpa_mgmt_dev *v_mdev, const char *name,
>>>>  	init_rwsem(&ndev->reslock);
>>>>  	config = &ndev->config;
>>>>  
>>>> +	/*
>>>> +	 * mlx5_vdpa vDPA devices currently don't support reporting or
>>>> +	 * setting the speed or duplex.
>>>> +	 */
>>>> +	config->speed  = cpu_to_mlx5vdpa32(mvdev, SPEED_UNKNOWN);
>>>> +	config->duplex = DUPLEX_UNKNOWN;
>>>> +
>>>>  	if (add_config->mask & BIT_ULL(VDPA_ATTR_DEV_NET_CFG_MTU)) {
>>>>  		err = config_func_mtu(mdev, add_config->net.mtu);
>>>>  		if (err)
>>>> -- 
>>>> 2.34.1