linux-kernel - Re: [RFC] vdpa/mlx5: preserve CVQ vringh index

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Date:   Thu, 26 Oct 2023 16:13:51 -0400
From:   Steven Sistare <steven.sistare@...cle.com>
To:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, Eli Cohen <elic@...dia.com>,
        Si-Wei Liu <si-wei.liu@...cle.com>,
        Dragos Tatulea <dtatulea@...dia.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [RFC] vdpa/mlx5: preserve CVQ vringh index

On 10/26/2023 4:11 PM, Steve Sistare wrote:
> mlx5_vdpa does not preserve userland's view of vring base for the control
> queue in the following sequence:
> 
> ioctl VHOST_SET_VRING_BASE
> ioctl VHOST_VDPA_SET_STATUS VIRTIO_CONFIG_S_DRIVER_OK
>   mlx5_vdpa_set_status()
>     setup_cvq_vring()
>       vringh_init_iotlb()
>         vringh_init_kern()
>           vrh->last_avail_idx = 0;
> ioctl VHOST_GET_VRING_BASE
> 
> To fix, restore the value of cvq->vring.last_avail_idx after calling
> vringh_init_iotlb.
> 
> Signed-off-by: Steve Sistare <steven.sistare@...cle.com>

This is a resend, I forgot to cc myself the first time.

I don't know if we expect vring_base to be preserved after reset, because the
uapi comments say nothing about it.  mlx5 *does* preserve base across reset
for the the data vq's, but perhaps that is an accident of the implementation.

I posted this patch to perhaps avoid future problems. The bug(?) bit me while
developing with an older version of qemu, and I can work around it in qemu
code.  Further, the latest version of qemu always enables svq for the cvq
and is not affected by this behavior AFAICT.

- Steve