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

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Date:   Thu, 25 Feb 2021 16:56:42 -0800
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, 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


Hi Michael,

Are you okay to live without this ioctl for now? I think QEMU is the one 
that needs to be fixed and will have to be made legacy guest aware. I 
think the kernel can just honor the feature negotiation result done by 
QEMU and do as what's told to.Will you agree?

If it's fine, I would proceed to reverting commit fe36cbe067 and related 
code in question from the kernel.

Thanks,
-Siwei

On 2/24/2021 10:24 AM, 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?
>
> One of the reason I asked is if this ioctl becomes a mandate for 
> vhost-vdpa kernel. QEMU would reject initialize vhost-vdpa if doesn't 
> see this ioctl coming?
>
> If it's optional, suppose the kernel may need it only when it becomes 
> necessary?
>
> Thanks,
> -Siwei