linux-kernel - Re: [PATCH 2/2] virtio: document virtio_config

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Message-Id: <20190103182849.7b0b0f0b@oc2783563651>
Date:   Thu, 3 Jan 2019 18:28:49 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, virtio-dev@...ts.oasis-open.org,
        Wei Wang <wei.w.wang@...el.com>
Subject: Re: [PATCH 2/2] virtio: document virtio_config_ops restrictions

On Thu,  3 Jan 2019 17:08:04 +0100
Cornelia Huck <cohuck@...hat.com> wrote:

> Some transports (e.g. virtio-ccw) implement virtio operations that
> seem to be a simple read/write as something more involved that
> cannot be done from an atomic context.
> 
> Give at least a hint about that.
> 
> Signed-off-by: Cornelia Huck <cohuck@...hat.com>
> ---
>  include/linux/virtio_config.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> index 7087ef946ba7..987b6491b946 100644
> --- a/include/linux/virtio_config.h
> +++ b/include/linux/virtio_config.h
> @@ -12,6 +12,11 @@ struct irq_affinity;
>  
>  /**
>   * virtio_config_ops - operations for configuring a virtio device
> + * Note: Do not assume that a transport implements all of the operations
> + *       getting/setting a value as a simple read/write! Generally speaking,
> + *       any of @get/@set, @get_status/@..._status, or @get_features/
> + *       @finalize_features are NOT safe to be called from an atomic
> + *       context.

I think the only exception is @bus_name (and maybe @generation, I don't
know) because it does not have to 'speak' with the hypervisor. If a
transport operation has to 'speak' with the hypervisor, we do it by
making it interpret a channel program. That means not safe to be called
form atomic context. Or am I missing something?

Regards,
Halil


>   * @get: read the value of a configuration field
>   *	vdev: the virtio_device
>   *	offset: the offset of the configuration field