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

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Message-ID: <f7479a55-9eee-4dec-8e09-ca01fa933112@nvidia.com>
Date: Tue, 27 Aug 2024 18:54:54 +0200
From: Dragos Tatulea <dtatulea@...dia.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Andrew Lunn <andrew@...n.ch>, Carlos Bilbao <cbilbao@...italocean.com>,
 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?



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.

Thanks,
Dragos