Virtualization - Mar 2021 - [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

On Thu, Feb 25, 2021 at 04:56:42PM -0800, Si-Wei Liu
wrote:
> 
> 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

Not really, I don't see why that's a good idea.  fe36cbe067 is the code
checking MTU before FEATURES_OK. Spec explicitly allows that.

-- 
MST

On 2/28/2021 1:27 PM, Michael S. Tsirkin wrote:
> On Thu, Feb 25, 2021 at 04:56:42PM -0800, Si-Wei Liu wrote:
>> 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
>
> Not really, I don't see why that's a good idea.  fe36cbe067 is the
code
> checking MTU before FEATURES_OK. Spec explicitly allows that.
>
Alright, but what I meant was this commit
452639a64ad8 ("vdpa: make sure set_features is invoked for legacy").

But I got why you need it in another email (for BE host/guest).

-Siwei