linux-kernel - Re: [RFC PATCH] vdpa: mandate 1.0 device

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Date:   Thu, 8 Apr 2021 11:59:56 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, parav@...dia.com, elic@...dia.com
Subject: Re: [RFC PATCH] vdpa: mandate 1.0 device

On Thu, Apr 08, 2021 at 04:26:48PM +0800, Jason Wang wrote:
> This patch mandates 1.0 for vDPA devices. The goal is to have the
> semantic of normative statement in the virtio spec and eliminate the
> burden of transitional device for both vDPA bus and vDPA parent.
> 
> uAPI seems fine since all the vDPA parent mandates
> VIRTIO_F_ACCESS_PLATFORM which implies 1.0 devices.
> 
> For legacy guests, it can still work since Qemu will mediate when
> necessary (e.g doing the endian conversion).
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>

Hmm. If we do this, don't we still have a problem with
legacy drivers which don't ack 1.0?
Note 1.0 affects ring endianness which is not mediated in QEMU
so QEMU can't pretend to device guest is 1.0.





> ---
>  include/linux/vdpa.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
> index 0fefeb976877..cfde4ec999b4 100644
> --- a/include/linux/vdpa.h
> +++ b/include/linux/vdpa.h
> @@ -6,6 +6,7 @@
>  #include <linux/device.h>
>  #include <linux/interrupt.h>
>  #include <linux/vhost_iotlb.h>
> +#include <uapi/linux/virtio_config.h>
>  
>  /**
>   * vDPA callback definition.
> @@ -317,6 +318,11 @@ static inline int vdpa_set_features(struct vdpa_device *vdev, u64 features)
>  {
>          const struct vdpa_config_ops *ops = vdev->config;
>  
> +        /* Mandating 1.0 to have semantics of normative statements in
> +         * the spec. */
> +        if (!(features & BIT_ULL(VIRTIO_F_VERSION_1)))
> +		return -EINVAL;
> +
>  	vdev->features_valid = true;
>          return ops->set_features(vdev, features);
>  }
> -- 
> 2.25.1