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

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

On 8/2/22 13:49, Vladimir Murzin wrote:
> 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. 

With the following diff FVP works agin

diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index a6f3a8a2ac..042503ad6c 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -54,6 +54,7 @@ static void request_entropy(struct virtrng_info *vi)
        reinit_completion(&vi->have_data);
        vi->data_avail = 0;
        vi->data_idx = 0;
+       smp_mb();
 
        sg_init_one(&sg, vi->data, sizeof(vi->data));
 

What do you reckon?

Cheers
Vladimir

> 
> Cheers
> Vladimir