linux-kernel - Re: [RFC] Why is set_config not supported in mlx5

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Message-ID: <41a2a533-549e-4f45-9d8d-68b5ef484b05@digitalocean.com>
Date: Wed, 28 Aug 2024 12:28:09 -0500
From: Carlos Bilbao <cbilbao@...italocean.com>
To: Dragos Tatulea <dtatulea@...dia.com>, Jason Wang <jasowang@...hat.com>
Cc: Andrew Lunn <andrew@...n.ch>, mst@...hat.com,
 virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org, eperezma@...hat.com,
 sashal@...nel.org, yuehaibing@...wei.com, steven.sistare@...cle.com
Subject: Re: [RFC] Why is set_config not supported in mlx5_vnet?

Hello,

On 8/27/24 11:54 AM, Dragos Tatulea wrote:
>
> On 27.08.24 04:03, Jason Wang wrote:
>> On Tue, Aug 27, 2024 at 12:11 AM Dragos Tatulea <dtatulea@...dia.com> wrote:
>>>
>>> On 26.08.24 16:24, Andrew Lunn wrote:
>>>> On Mon, Aug 26, 2024 at 11:06:09AM +0200, Dragos Tatulea wrote:
>>>>>
>>>>> On 23.08.24 18:54, Carlos Bilbao wrote:
>>>>>> Hello,
>>>>>>
>>>>>> I'm debugging my vDPA setup, and when using ioctl to retrieve the
>>>>>> configuration, I noticed that it's running in half duplex mode:
>>>>>>
>>>>>> Configuration data (24 bytes):
>>>>>>   MAC address: (Mac address)
>>>>>>   Status: 0x0001
>>>>>>   Max virtqueue pairs: 8
>>>>>>   MTU: 1500
>>>>>>   Speed: 0 Mb
>>>>>>   Duplex: Half Duplex
>>>>>>   RSS max key size: 0
>>>>>>   RSS max indirection table length: 0
>>>>>>   Supported hash types: 0x00000000
>>>>>>
>>>>>> I believe this might be contributing to the underperformance of vDPA.
>>>>> mlx5_vdpa vDPA devicess currently do not support the VIRTIO_NET_F_SPEED_DUPLEX
>>>>> feature which reports speed and duplex. You can check the state on the
>>>>> PF.
>>>> Then it should probably report DUPLEX_UNKNOWN.
>>>>
>>>> The speed of 0 also suggests SPEED_UNKNOWN is not being returned. So
>>>> this just looks buggy in general.
>>>>
>>> The virtio spec doesn't mention what those values should be when
>>> VIRTIO_NET_F_SPEED_DUPLEX is not supported.
>>>
>>> Jason, should vdpa_dev_net_config_fill() initialize the speed/duplex
>>> fields to SPEED/DUPLEX_UNKNOWN instead of 0?
>> Spec said
>>
>> """
>> The following two fields, speed and duplex, only exist if
>> VIRTIO_NET_F_SPEED_DUPLEX is set.
>> """
>>
>> So my understanding is that it is undefined behaviour, and those
>> fields seems useless before feature negotiation. For safety, it might
>> be better to initialize them as UNKOWN.
>>
> After a closer look my statement doesn't make sense: the device will copy
> the virtio_net_config bytes on top.
>
> The solution is to initialize these fields to UNKNOWN in the driver. Will send
> a patch to fix this.


With Dragos' permission, I'm sending a first draft of this now.


>
> Thanks,
> Dragos


Thanks, Carlos