Re: [PATCH linux-next 1/9] vdpa_sim: Consider read only supported features instead of current

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Feb 24, 2021 at 08:18:36AM +0200, Parav Pandit wrote:
> To honor VIRTIO_F_VERSION_1 feature bit, during endianness detection,
> consider the read only supported features bit instead of current
> features bit which can be modified by the driver.
> 
> This enables vdpa_sim_net driver to invoke cpu_to_vdpasim16() early
> enough just after vdpasim device creation in subsequent patch.
> 
> Signed-off-by: Parav Pandit <parav@nvidia.com>
> Reviewed-by: Eli Cohen <elic@nvidia.com>

Well that works for legacy and modern devices but not for transitional
ones. Without transitional device support vendors are reluctant to add
modern features since that will break old guests ...  I suspect we need
to either add a new ioctl enabling modern mode, or abuse SET_FEATURES
and call it from qemu on first config space access.

> ---
>  drivers/vdpa/vdpa_sim/vdpa_sim.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.h b/drivers/vdpa/vdpa_sim/vdpa_sim.h
> index 6d75444f9948..176d641a0939 100644
> --- a/drivers/vdpa/vdpa_sim/vdpa_sim.h
> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.h
> @@ -11,6 +11,7 @@
>  #include <linux/virtio_byteorder.h>
>  #include <linux/vhost_iotlb.h>
>  #include <uapi/linux/virtio_config.h>
> +#include <linux/bits.h>
>  
>  #define VDPASIM_FEATURES	((1ULL << VIRTIO_F_ANY_LAYOUT) | \
>  				 (1ULL << VIRTIO_F_VERSION_1)  | \
> @@ -71,7 +72,8 @@ struct vdpasim *vdpasim_create(struct vdpasim_dev_attr *attr);
>  static inline bool vdpasim_is_little_endian(struct vdpasim *vdpasim)
>  {
>  	return virtio_legacy_is_little_endian() ||
> -		(vdpasim->features & (1ULL << VIRTIO_F_VERSION_1));
> +		(vdpasim->dev_attr.supported_features &
> +		 BIT_ULL(VIRTIO_F_VERSION_1));
>  }
>  
>  static inline u16 vdpasim16_to_cpu(struct vdpasim *vdpasim, __virtio16 val)
> -- 
> 2.26.2

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