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

[Jason Wang]

On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > 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
> Overall I would be fine with this approach but we need to avoid breaking
> working userspace, qemu releases with vdpa support are out there and
> seem to work for people. Any changes need to take that into account
> and document compatibility concerns.


Agree, let me check.


>   I note that any hardware
> implementation is already broken for legacy except on platforms with
> strong ordering which might be helpful in reducing the scope.


Yes.

Thanks


>