linux-kernel - Re: [PATCH] vdpa/mlx5: Fix mlx5_vdpa_get_config() endianness on big-endian machines.

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Message-ID: <klvpj7l5tbrwpjxfch7rgy2abqxbi6riuhvoxgg6whbbjgrf5q@wkaamsogcp7e>
Date: Fri, 31 Jan 2025 08:57:40 +0100
From: Dragos Tatulea <dtatulea@...dia.com>
To: Konstantin Shkolnyy <kshk@...ux.ibm.com>
Cc: virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	mjrosato@...ux.ibm.com, mst@...hat.com, jasowang@...hat.com
Subject: Re: [PATCH] vdpa/mlx5: Fix mlx5_vdpa_get_config() endianness on
 big-endian machines.

On 01/30, Konstantin Shkolnyy wrote:
> mlx5_vdpa_dev_add() doesn’t initialize mvdev->actual_features. It’s
> initialized later by mlx5_vdpa_set_driver_features(). However,
> mlx5_vdpa_get_config() depends on the VIRTIO_F_VERSION_1 flag in
> actual_features to return data with correct endianness. When it’s called
> before mlx5_vdpa_set_driver_features(), the flag is clear, and the data are
> returned as big-endian on big-endian machines, while QEMU interprets them
> as little-endian.
>
Good catch! Thank you for the patch.

Could you add the clarification that mlx5_vdpa_dev_add() uses the
endianess helpers (cpu_to_mlx5vdpa16()) which depend on actual_flags
being set properly.

> Signed-off-by: Konstantin Shkolnyy <kshk@...ux.ibm.com>
> ---
>  drivers/vdpa/mlx5/net/mlx5_vnet.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> index 36099047560d..f9ea240b8145 100644
> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> @@ -3884,6 +3884,7 @@ static int mlx5_vdpa_dev_add(struct vdpa_mgmt_dev *v_mdev, const char *name,
>  	ndev->mvdev.max_vqs = max_vqs;
>  	mvdev = &ndev->mvdev;
>  	mvdev->mdev = mdev;
> +	mvdev->actual_features = device_features;
>
Maybe it is better to be conservative: only pick the VIRTIO_F_VERSION_1
flag out of device_features.

Thanks,
Dragos