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

Michael S. Tsirkin
2021-09-23T07:04:34Z

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

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

> >
> > > /* 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.

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MST