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:

quoted

quoted

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