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

[Michael S. Tsirkin]

On Wed, Aug 03, 2022 at 09:57:35AM +0100, Vladimir Murzin wrote:
> 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@xxxxxxxxxx>
> >> ---
> >>  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

Thanks for debugging this!

OK, interesting.

data_idx and data_avail are accessed from virtio_read.

Which as far as I can tell is invoked just with reading_mutex.


But, request_entropy is called from probe when device is registered
this time without locks
so it can trigger while another thread is calling virtio_read.

Second request_entropy is called from a callback random_recv_done
also without locks.

So it's great that smp_mb helped here but I suspect in fact we
need locking. Laurent?

-- 
MST