On 8/3/22 12:39, Michael S. Tsirkin wrote:
> 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?
>
I'm sorry for the noise, but it looks like I'm seeing issue for some different reasons.
I manage to reproduce issue even with smb_mb() in place. The reason I though it helped
is because I changed both environment and added smb_mb().
Anyway, thank you for your time!
Cheers
Vladimir
