Date:   Wed, 10 Aug 2022 08:54:24 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Laurent Vivier <lvivier@...hat.com>, linux-kernel@...r.kernel.org,
        amit@...nel.org, Herbert Xu <herbert@...dor.apana.org.au>,
        Matt Mackall <mpm@...enic.com>,
        virtualization@...ts.linux-foundation.org,
        Dmitriy Vyukov <dvyukov@...gle.com>, rusty@...tcorp.com.au,
        akong@...hat.com, Alexander Potapenko <glider@...gle.com>,
        linux-crypto@...r.kernel.org,
        Mauricio De Carvalho <Mauricio.DeCarvalho@....com>,
        Kevin Brodsky <Kevin.Brodsky@....com>
Subject: Re: [PATCH v2 4/4] hwrng: virtio - always add a pending request

On 8/9/22 21:16, Michael S. Tsirkin wrote:
> On Wed, Aug 03, 2022 at 02:12:25PM +0100, Vladimir Murzin wrote:
>> On 8/3/22 13:55, Michael S. Tsirkin wrote:
>>> On Wed, Aug 03, 2022 at 01:25:15PM +0100, Vladimir Murzin wrote:
>>>> 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@...hat.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
>>>>>
>>>>> 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
>>>
>>> Well we at least have a race condition found by code review here. Here's
>>> a quick hack attempting to fix it. I don't like it much since
>>> it adds buffers with GFP_ATOMIC and kicks under a spinlock, but
>>> for now we can at least test it. I did a quick build but that's
>>> all, a bit rushed now sorry. Would appreciate knowing whether
>>> this addresses the issue for you.
>>>
>>>
>>> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
>>> index a6f3a8a2aca6..36121c8d0315 100644
>>> --- a/drivers/char/hw_random/virtio-rng.c
>>> +++ b/drivers/char/hw_random/virtio-rng.c
>>> @@ -23,6 +23,7 @@ struct virtrng_info {
>>>       bool hwrng_register_done;
>>>       bool hwrng_removed;
>>>       /* data transfer */
>>> +     spinlock_t lock;
>>>       struct completion have_data;
>>>       unsigned int data_avail;
>>>       unsigned int data_idx;
>>> @@ -37,6 +38,9 @@ struct virtrng_info {
>>>  static void random_recv_done(struct virtqueue *vq)
>>>  {
>>>       struct virtrng_info *vi = vq->vdev->priv;
>>> +     unsigned long flags;
>>> +
>>> +     spin_lock_irqsave(&vi->lock, flags);
>>>
>>>       /* We can get spurious callbacks, e.g. shared IRQs + virtio_pci. */
>>>       if (!virtqueue_get_buf(vi->vq, &vi->data_avail))
>>> @@ -45,20 +49,20 @@ static void random_recv_done(struct virtqueue *vq)
>>>       vi->data_idx = 0;
>>>
>>>       complete(&vi->have_data);
>>> +     spin_unlock_irqrestore(&vi->lock, flags);
>>>  }
>>>
>>>  static void request_entropy(struct virtrng_info *vi)
>>>  {
>>>       struct scatterlist sg;
>>>
>>> -     reinit_completion(&vi->have_data);
>>> -     vi->data_avail = 0;
>>> +     BUG_ON(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. */
>>> -     virtqueue_add_inbuf(vi->vq, &sg, 1, vi->data, GFP_KERNEL);
>>> +     virtqueue_add_inbuf(vi->vq, &sg, 1, vi->data, GFP_ATOMIC);
>>>
>>>       virtqueue_kick(vi->vq);
>>>  }
>>> @@ -70,8 +74,10 @@ 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)
>>> +     if (vi->data_avail == 0) {
>>> +             reinit_completion(&vi->have_data);
>>>               request_entropy(vi);
>>> +     }
>>>       return size;
>>>  }
>>>
>>> @@ -81,18 +87,21 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
>>>       struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
>>>       unsigned int chunk;
>>>       size_t read;
>>> +     unsigned long flags;
>>>
>>>       if (vi->hwrng_removed)
>>>               return -ENODEV;
>>>
>>>       read = 0;
>>>
>>> +     spin_lock_irqsave(&vi->lock, flags);
>>>       /* copy available data */
>>>       if (vi->data_avail) {
>>>               chunk = copy_data(vi, buf, size);
>>>               size -= chunk;
>>>               read += chunk;
>>>       }
>>> +     spin_unlock_irqrestore(&vi->lock, flags);
>>>
>>>       if (!wait)
>>>               return read;
>>> @@ -108,12 +117,14 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
>>>               /* if vi->data_avail is 0, we have been interrupted
>>>                * by a cleanup, but buffer stays in the queue
>>>                */
>>> +             spin_lock_irqsave(&vi->lock, flags);
>>>               if (vi->data_avail == 0)
>>>                       return read;
>>>
>>>               chunk = copy_data(vi, buf + read, size);
>>>               size -= chunk;
>>>               read += chunk;
>>> +             spin_unlock_irqrestore(&vi->lock, flags);
>>>       }
>>>
>>>       return read;
>>> @@ -122,19 +133,25 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
>>>  static void virtio_cleanup(struct hwrng *rng)
>>>  {
>>>       struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
>>> +     unsigned long flags;
>>>
>>> +     spin_lock_irqsave(&vi->lock, flags);
>>>       complete(&vi->have_data);
>>> +     spin_unlock_irqrestore(&vi->lock, flags);
>>>  }
>>>
>>>  static int probe_common(struct virtio_device *vdev)
>>>  {
>>>       int err, index;
>>>       struct virtrng_info *vi = NULL;
>>> +     unsigned long flags;
>>>
>>>       vi = kzalloc(sizeof(struct virtrng_info), GFP_KERNEL);
>>>       if (!vi)
>>>               return -ENOMEM;
>>>
>>> +     spin_lock_init(&vi->lock);
>>> +
>>>       vi->index = index = ida_simple_get(&rng_index_ida, 0, 0, GFP_KERNEL);
>>>       if (index < 0) {
>>>               err = index;
>>> @@ -162,7 +179,10 @@ static int probe_common(struct virtio_device *vdev)
>>>       virtio_device_ready(vdev);
>>>
>>>       /* we always have a pending entropy request */
>>> -     request_entropy(vi);
>>> +     spin_lock_irqsave(&vi->lock, flags);
>>> +     if (vi->data_avail == 0)
>>> +             request_entropy(vi);
>>> +     spin_unlock_irqrestore(&vi->lock, flags);
>>>
>>>       return 0;
>>>
>>>
>>
>> Thanks a lot! I gave it a go and it did not help. I think I need to find out how exactly
>> my environment affected... it not necessary need to be kernel related.
> 
> Okay ... I'll wait to hear your report then. You are saying maybe
> there's no bug in kernel, something else changed in your environment?
> 

Yes, I noticed that if I swap bootloader then problem goes away, so unlikely
kernel issue. I passed reproducer to people working on a model, so they can
investigate from the other side. So far feel free to ignore my report. 

Cheers
Vladimir

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