linux-kernel - Re: [PATCH v2 4/7] virtio: VIRTIO_DEVICE_F

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Date:   Thu, 2 Feb 2023 12:29:54 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
Cc:     jasowang@...hat.com, parav@...dia.com, elic@...dia.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/7] virtio: VIRTIO_DEVICE_F_MASK for all per-device
 features

On Tue, Jan 31, 2023 at 03:22:22PM -0800, Si-Wei Liu wrote:
> Introduce VIRTIO_DEVICE_F_MASK bitmask used for identification
> of per-device features. Feature bits VIRTIO_TRANSPORT_F_START
> through VIRTIO_TRANSPORT_F_END are reserved for transport
> features hence are not counted as per-device features against
> the 64bit feature space.
> 
> Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
> ---
>  include/uapi/linux/virtio_config.h | 8 ++++++++
>  1 file changed, 8 insertions(+)

Pls don't add this in uapi, people tend to depend on this and then
things fail when we extend virtio. For example this won't work with >64
feature bits.

> 
> diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
> index 3c05162..3bdc7ed 100644
> --- a/include/uapi/linux/virtio_config.h
> +++ b/include/uapi/linux/virtio_config.h
> @@ -54,6 +54,14 @@
>  #define VIRTIO_TRANSPORT_F_START	28
>  #define VIRTIO_TRANSPORT_F_END		41
>  
> +/*
> + * Bitmask for all per-device features: feature bits VIRTIO_TRANSPORT_F_START
> + * through VIRTIO_TRANSPORT_F_END are unset, i.e. 0xfffffc000fffffff for
> + * all 64bit features
> + */
> +#define VIRTIO_DEVICE_F_MASK (~0ULL << (VIRTIO_TRANSPORT_F_END + 1) | \
> +			      ((1ULL << VIRTIO_TRANSPORT_F_START) - 1))
> +
>  #ifndef VIRTIO_CONFIG_NO_LEGACY
>  /* Do we get callbacks when the ring is completely used, even if we've
>   * suppressed them? */
> -- 
> 1.8.3.1