Re: [PATCH 1/4] hwrng: virtio - add an internal buffer

Laurent Vivier
2021-09-23T06:26:14Z

On 22/09/2021 21:02, Michael S. Tsirkin wrote:
> On Wed, Sep 22, 2021 at 07:09:00PM +0200, Laurent Vivier wrote:
>> hwrng core uses two buffers that can be mixed in the
>> virtio-rng queue.
>>
>> If the buffer is provided with wait=0 it is enqueued in the
>> virtio-rng queue but unused by the caller.
>> On the next call, core provides another buffer but the
>> first one is filled instead and the new one queued.
>> And the caller reads the data from the new one that is not
>> updated, and the data in the first one are lost.
>>
>> To avoid this mix, virtio-rng needs to use its own unique
>> internal buffer at a cost of a data copy to the caller buffer.
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>> ---
>> drivers/char/hw_random/virtio-rng.c | 43 ++++++++++++++++++++++-------
>> 1 file changed, 33 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
>> index a90001e02bf7..208c547dcac1 100644
>> --- a/drivers/char/hw_random/virtio-rng.c
>> +++ b/drivers/char/hw_random/virtio-rng.c
>> @@ -18,13 +18,20 @@ static DEFINE_IDA(rng_index_ida);
>> struct virtrng_info {
>> struct hwrng hwrng;
>> struct virtqueue *vq;
>> - struct completion have_data;
>> char name[25];
>> - unsigned int data_avail;
>> int index;
>> bool busy;
>> bool hwrng_register_done;
>> bool hwrng_removed;
>> + /* data transfer */
>> + struct completion have_data;
>> + unsigned int data_avail;
>> + /* minimal size returned by rng_buffer_size() */
>> +#if SMP_CACHE_BYTES < 32
>> + u8 data[32];
>> +#else
>> + u8 data[SMP_CACHE_BYTES];
>> +#endif
>
> Let's move this logic to a macro in hw_random.h ?
>
>> };
>>
>> static void random_recv_done(struct virtqueue *vq)
>> @@ -39,14 +46,14 @@ static void random_recv_done(struct virtqueue *vq)
>> }
>>
>> /* The host will fill any buffer we give it with sweet, sweet randomness. */
>> -static void register_buffer(struct virtrng_info *vi, u8 *buf, size_t size)
>> +static void register_buffer(struct virtrng_info *vi)
>> {
>> struct scatterlist sg;
>>
>> - sg_init_one(&sg, buf, size);
>> + sg_init_one(&sg, vi->data, sizeof(vi->data));
>
> Note that add_early_randomness requests less:
> size_t size = min_t(size_t, 16, rng_buffer_size());
>
> maybe track how much was requested and grow up to sizeof(data)?

I think this problem is managed by PATCH 3/4 as we reuse unused data of the buffer.

>
>>
>> /* There should always be room for one buffer. */
>> - virtqueue_add_inbuf(vi->vq, &sg, 1, buf, GFP_KERNEL);
>> + virtqueue_add_inbuf(vi->vq, &sg, 1, vi->data, GFP_KERNEL);
>
>
> BTW no longer true if DMA API is in use ... not easy to fix,
> I think some changes to virtio API to allow pre-mapping
> s/g for DMA might be needed ...

Is there something I can do here?

Thanks,
Laurent