On Thu, Sep 23, 2021 at 08:26:06AM +0200, Laurent Vivier wrote:

On 22/09/2021 21:02, Michael S. Tsirkin wrote:

quoted

On Wed, Sep 22, 2021 at 07:09:00PM +0200, Laurent Vivier wrote:

quoted

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 <redacted>
---
  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 ?

quoted

  };
  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.

the issue I'm pointing out is that we are requesting too much
entropy from host - more than guest needs.

quoted

quoted

  	/* 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

We can let it be for now, but down the road I think we should
support a way to pre-map memory for DMA.

-- 
MST

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