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

[Laurent Vivier]

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@xxxxxxxxxx>
> > ---
> >   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