linux-kernel - Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero

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Message-ID: <2cb51a6d-afa0-7cd1-d6f2-6b153186eaca@redhat.com>
Date:   Mon, 1 Mar 2021 11:56:50 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Si-Wei Liu <si-wei.liu@...cle.com>
Cc:     elic@...dia.com, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero


On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>> the kernel that it's a legacy guest. Ugh :(
>> Well, although I think adding an ioctl is doable, may I know what the use
>> case there will be for kernel to leverage such info directly? Is there a
>> case QEMU can't do with dedicate ioctls later if there's indeed
>> differentiation (legacy v.s. modern) needed?
> BTW a good API could be
>
> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>
> we did it per vring but maybe that was a mistake ...


Actually, I wonder whether it's good time to just not support legacy 
driver for vDPA. Consider:

1) It's definition is no-normative
2) A lot of budren of codes

So qemu can still present the legacy device since the config space or 
other stuffs that is presented by vhost-vDPA is not expected to be 
accessed by guest directly. Qemu can do the endian conversion when 
necessary in this case?

Thanks


>