Re: [PATCH 1/2] vdpa/mlx5: Make VIRTIO_NET_F_MRG_RXBUF off by default

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Parav Pandit <parav@mellanox.com>
Cc: "perezma@redhat.com" <perezma@redhat.com>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>,
	Eugenio Perez Martin <eperezma@redhat.com>,
	Eli Cohen <elic@nvidia.com>
Subject: Re: [PATCH 1/2] vdpa/mlx5: Make VIRTIO_NET_F_MRG_RXBUF off by default
Date: Mon, 13 Mar 2023 23:37:59 -0400	[thread overview]
Message-ID: <20230313233638-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <PH0PR12MB5481A0629C479A030C8A8EDEDCBE9@PH0PR12MB5481.namprd12.prod.outlook.com>

On Tue, Mar 14, 2023 at 02:05:50AM +0000, Parav Pandit wrote:
> 
> > From: Si-Wei Liu <si-wei.liu@oracle.com>
> > Sent: Monday, March 13, 2023 6:19 PM
> 
> > Actually there's no such burden or requirement to maintain backward
> > compatibility for the default 'vdpa dev add' behavior if dedicated
> > device_features is not specified. Historically the default vdpa creation on mlx5
> > ever got changed from single queue to 8 queue pairs when VIRTIO_NET_F_MQ
> > feature was first introduced to mlx5_vdpa, then the default switched back to 1
> > data queue pair again when max_vqp attribute was added to the vdpa tool.
> > Essentially, every addition of new feature to mlx5_vdpa, e.g. CTRL_VQ,
> > CTRL_VLAN, and et al, effectively changed the default "vdpa dev add" behavior
> > not just only once: the backward compatibility guarantee is simply just not
> > there and ever.
> This requires that every change in the device attributes will change the behavior for vdpa dev add.
> The OR operation of the user supplied feature bits with device defaults feature bit doesn’t look good to me.
> It brings uncertain behavior.
> 
> The right behavior should be, if user supplied the feature bits, it should supply all desired bits.

I think u mean all that device also supports.

> If user doesn’t supply feature bits, use the device defaults.
> But not to mix both feature bits.

Can't say I understand what this means. What does "both" mean here?

-- 
MST

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