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

Michael S. Tsirkin
2021-09-22T19:02:18Z

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)?

>
> /* 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 ...

>
> virtqueue_kick(vi->vq);
> }
> @@ -55,6 +62,8 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
> {
> int ret;
> struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
> + unsigned int chunk;
> + size_t read;
>
> if (vi->hwrng_removed)
> return -ENODEV;
> @@ -62,19 +71,33 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
> if (!vi->busy) {
> vi->busy = true;
> reinit_completion(&vi->have_data);
> - register_buffer(vi, buf, size);
> + register_buffer(vi);
> }
>
> if (!wait)
> return 0;
>
> - ret = wait_for_completion_killable(&vi->have_data);
> - if (ret < 0)
> - return ret;
> + read = 0;
> + while (size != 0) {
> + ret = wait_for_completion_killable(&vi->have_data);
> + if (ret < 0)
> + return ret;
> +
> + chunk = min_t(unsigned int, size, vi->data_avail);
> + memcpy(buf + read, vi->data, chunk);
> + read += chunk;
> + size -= chunk;
> + vi->data_avail = 0;
> +
> + if (size != 0) {
> + reinit_completion(&vi->have_data);
> + register_buffer(vi);
> + }
> + }
>
> vi->busy = false;
>
> - return vi->data_avail;
> + return read;
> }
>
> static void virtio_cleanup(struct hwrng *rng)
> --
> 2.31.1