Re: [PATCH v2 4/4] hwrng: virtio - always add a pending request

[Vladimir Murzin <vladimir.murzin@arm.com>]

Hi Laurent,

On 10/28/21 11:11, Laurent Vivier wrote:
> If we ensure we have already some data available by enqueuing
> again the buffer once data are exhausted, we can return what we
> have without waiting for the device answer.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  drivers/char/hw_random/virtio-rng.c | 26 ++++++++++++--------------
>  1 file changed, 12 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
> index 8ba97cf4ca8f..0a7dde135db1 100644
> --- a/drivers/char/hw_random/virtio-rng.c
> +++ b/drivers/char/hw_random/virtio-rng.c
> @@ -20,7 +20,6 @@ struct virtrng_info {
>  	struct virtqueue *vq;
>  	char name[25];
>  	int index;
> -	bool busy;
>  	bool hwrng_register_done;
>  	bool hwrng_removed;
>  	/* data transfer */
> @@ -44,16 +43,18 @@ static void random_recv_done(struct virtqueue *vq)
>  		return;
>  
>  	vi->data_idx = 0;
> -	vi->busy = false;
>  
>  	complete(&vi->have_data);
>  }
>  
> -/* The host will fill any buffer we give it with sweet, sweet randomness. */
> -static void register_buffer(struct virtrng_info *vi)
> +static void request_entropy(struct virtrng_info *vi)
>  {
>  	struct scatterlist sg;
>  
> +	reinit_completion(&vi->have_data);
> +	vi->data_avail = 0;
> +	vi->data_idx = 0;
> +
>  	sg_init_one(&sg, vi->data, sizeof(vi->data));
>  
>  	/* There should always be room for one buffer. */
> @@ -69,6 +70,8 @@ static unsigned int copy_data(struct virtrng_info *vi, void *buf,
>  	memcpy(buf, vi->data + vi->data_idx, size);
>  	vi->data_idx += size;
>  	vi->data_avail -= size;
> +	if (vi->data_avail == 0)
> +		request_entropy(vi);
>  	return size;
>  }
>  
> @@ -98,13 +101,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
>  	 * so either size is 0 or data_avail is 0
>  	 */
>  	while (size != 0) {
> -		/* data_avail is 0 */
> -		if (!vi->busy) {
> -			/* no pending request, ask for more */
> -			vi->busy = true;
> -			reinit_completion(&vi->have_data);
> -			register_buffer(vi);
> -		}
> +		/* data_avail is 0 but a request is pending */
>  		ret = wait_for_completion_killable(&vi->have_data);
>  		if (ret < 0)
>  			return ret;
> @@ -126,8 +123,7 @@ static void virtio_cleanup(struct hwrng *rng)
>  {
>  	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
>  
> -	if (vi->busy)
> -		complete(&vi->have_data);
> +	complete(&vi->have_data);
>  }
>  
>  static int probe_common(struct virtio_device *vdev)
> @@ -163,6 +159,9 @@ static int probe_common(struct virtio_device *vdev)
>  		goto err_find;
>  	}
>  
> +	/* we always have a pending entropy request */
> +	request_entropy(vi);
> +
>  	return 0;
>  
>  err_find:
> @@ -181,7 +180,6 @@ static void remove_common(struct virtio_device *vdev)
>  	vi->data_idx = 0;
>  	complete(&vi->have_data);
>  	vdev->config->reset(vdev);
> -	vi->busy = false;
>  	if (vi->hwrng_register_done)
>  		hwrng_unregister(&vi->hwrng);
>  	vdev->config->del_vqs(vdev);

We observed that after this commit virtio-rng implementation in FVP doesn't
work

INFO: bp.virtio_rng: Selected Random Generator Device: XORSHIFT DEVICE
INFO: bp.virtio_rng: Using seed value: 0x5674bba8
Error: FVP_Base_AEMvA: bp.virtio_rng: <vq0-requestq> Found invalid descriptor index
In file: (unknown):0
In process: FVP_Base_AEMvA.thread_p_12 @ 935500020 ns
Info: FVP_Base_AEMvA: bp.virtio_rng: Could not extract buffer

while basic baremetal test works as expected

INFO: bp.virtio_rng: Selected Random Generator Device: XORSHIFT DEVICE
INFO: bp.virtio_rng: Using seed value: 0x541c142e
Info: FVP_Base_AEMv8A: bp.virtio_rng: Generated Number: 0x4b098991ceb377e6
Info: FVP_Base_AEMv8A: bp.virtio_rng: Generated Number: 0xbdcbe3f765ba62f7

We are trying to get an idea what is missing and where, yet none of us familiar
with the driver :(

I'm looping Kevin who originally reported that and Mauricio who is looking form
the FVP side. 

Cheers
Vladimir