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

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Message-ID: <CACGkMEvUeHTKu8+=sPLz-ddr3xb7dD29bwNex=xvyRdK1gBrgw@mail.gmail.com>
Date: Wed, 28 Aug 2024 09:52:39 +0800
From: Jason Wang <jasowang@...hat.com>
To: Dragos Tatulea <dtatulea@...dia.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 Wed, Aug 28, 2024 at 12:55 AM Dragos Tatulea <dtatulea@...dia.com> 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.
>

For "driver", I guess you meant virtio-net?

Thanks

> Thanks,
> Dragos
>