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Date: Wed, 3 Aug 2022 07:39:57 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Vladimir Murzin <vladimir.murzin@....com>
Cc: Laurent Vivier <lvivier@...hat.com>, linux-kernel@...r.kernel.org,
amit@...nel.org, Herbert Xu <herbert@...dor.apana.org.au>,
Matt Mackall <mpm@...enic.com>,
virtualization@...ts.linux-foundation.org,
Dmitriy Vyukov <dvyukov@...gle.com>, rusty@...tcorp.com.au,
akong@...hat.com, Alexander Potapenko <glider@...gle.com>,
linux-crypto@...r.kernel.org,
Mauricio De Carvalho <Mauricio.DeCarvalho@....com>,
Kevin Brodsky <Kevin.Brodsky@....com>
Subject: Re: [PATCH v2 4/4] hwrng: virtio - always add a pending request
On Wed, Aug 03, 2022 at 09:57:35AM +0100, Vladimir Murzin wrote:
> On 8/2/22 13:49, Vladimir Murzin wrote:
> > Hi Laurent,
> >
> > On 10/28/21 11:11, Laurent Vivier wrote:
> >> If we ensure we have already some data available by enqueuing
> >> again the buffer once data are exhausted, we can return what we
> >> have without waiting for the device answer.
> >>
> >> Signed-off-by: Laurent Vivier <lvivier@...hat.com>
> >> ---
> >> drivers/char/hw_random/virtio-rng.c | 26 ++++++++++++--------------
> >> 1 file changed, 12 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
> >> index 8ba97cf4ca8f..0a7dde135db1 100644
> >> --- a/drivers/char/hw_random/virtio-rng.c
> >> +++ b/drivers/char/hw_random/virtio-rng.c
> >> @@ -20,7 +20,6 @@ struct virtrng_info {
> >> struct virtqueue *vq;
> >> char name[25];
> >> int index;
> >> - bool busy;
> >> bool hwrng_register_done;
> >> bool hwrng_removed;
> >> /* data transfer */
> >> @@ -44,16 +43,18 @@ static void random_recv_done(struct virtqueue *vq)
> >> return;
> >>
> >> vi->data_idx = 0;
> >> - vi->busy = false;
> >>
> >> complete(&vi->have_data);
> >> }
> >>
> >> -/* The host will fill any buffer we give it with sweet, sweet randomness. */
> >> -static void register_buffer(struct virtrng_info *vi)
> >> +static void request_entropy(struct virtrng_info *vi)
> >> {
> >> struct scatterlist sg;
> >>
> >> + reinit_completion(&vi->have_data);
> >> + vi->data_avail = 0;
> >> + vi->data_idx = 0;
> >> +
> >> sg_init_one(&sg, vi->data, sizeof(vi->data));
> >>
> >> /* There should always be room for one buffer. */
> >> @@ -69,6 +70,8 @@ static unsigned int copy_data(struct virtrng_info *vi, void *buf,
> >> memcpy(buf, vi->data + vi->data_idx, size);
> >> vi->data_idx += size;
> >> vi->data_avail -= size;
> >> + if (vi->data_avail == 0)
> >> + request_entropy(vi);
> >> return size;
> >> }
> >>
> >> @@ -98,13 +101,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
> >> * so either size is 0 or data_avail is 0
> >> */
> >> while (size != 0) {
> >> - /* data_avail is 0 */
> >> - if (!vi->busy) {
> >> - /* no pending request, ask for more */
> >> - vi->busy = true;
> >> - reinit_completion(&vi->have_data);
> >> - register_buffer(vi);
> >> - }
> >> + /* data_avail is 0 but a request is pending */
> >> ret = wait_for_completion_killable(&vi->have_data);
> >> if (ret < 0)
> >> return ret;
> >> @@ -126,8 +123,7 @@ static void virtio_cleanup(struct hwrng *rng)
> >> {
> >> struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
> >>
> >> - if (vi->busy)
> >> - complete(&vi->have_data);
> >> + complete(&vi->have_data);
> >> }
> >>
> >> static int probe_common(struct virtio_device *vdev)
> >> @@ -163,6 +159,9 @@ static int probe_common(struct virtio_device *vdev)
> >> goto err_find;
> >> }
> >>
> >> + /* we always have a pending entropy request */
> >> + request_entropy(vi);
> >> +
> >> return 0;
> >>
> >> err_find:
> >> @@ -181,7 +180,6 @@ static void remove_common(struct virtio_device *vdev)
> >> vi->data_idx = 0;
> >> complete(&vi->have_data);
> >> vdev->config->reset(vdev);
> >> - vi->busy = false;
> >> if (vi->hwrng_register_done)
> >> hwrng_unregister(&vi->hwrng);
> >> vdev->config->del_vqs(vdev);
> >
> > We observed that after this commit virtio-rng implementation in FVP doesn't
> > work
> >
> > INFO: bp.virtio_rng: Selected Random Generator Device: XORSHIFT DEVICE
> > INFO: bp.virtio_rng: Using seed value: 0x5674bba8
> > Error: FVP_Base_AEMvA: bp.virtio_rng: <vq0-requestq> Found invalid descriptor index
> > In file: (unknown):0
> > In process: FVP_Base_AEMvA.thread_p_12 @ 935500020 ns
> > Info: FVP_Base_AEMvA: bp.virtio_rng: Could not extract buffer
> >
> > while basic baremetal test works as expected
> >
> > INFO: bp.virtio_rng: Selected Random Generator Device: XORSHIFT DEVICE
> > INFO: bp.virtio_rng: Using seed value: 0x541c142e
> > Info: FVP_Base_AEMv8A: bp.virtio_rng: Generated Number: 0x4b098991ceb377e6
> > Info: FVP_Base_AEMv8A: bp.virtio_rng: Generated Number: 0xbdcbe3f765ba62f7
> >
> > We are trying to get an idea what is missing and where, yet none of us familiar
> > with the driver :(
> >
> > I'm looping Kevin who originally reported that and Mauricio who is looking form
> > the FVP side.
>
> With the following diff FVP works agin
>
> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
> index a6f3a8a2ac..042503ad6c 100644
> --- a/drivers/char/hw_random/virtio-rng.c
> +++ b/drivers/char/hw_random/virtio-rng.c
> @@ -54,6 +54,7 @@ static void request_entropy(struct virtrng_info *vi)
> reinit_completion(&vi->have_data);
> vi->data_avail = 0;
> vi->data_idx = 0;
> + smp_mb();
>
> sg_init_one(&sg, vi->data, sizeof(vi->data));
>
>
> What do you reckon?
>
> Cheers
> Vladimir
Thanks for debugging this!
OK, interesting.
data_idx and data_avail are accessed from virtio_read.
Which as far as I can tell is invoked just with reading_mutex.
But, request_entropy is called from probe when device is registered
this time without locks
so it can trigger while another thread is calling virtio_read.
Second request_entropy is called from a callback random_recv_done
also without locks.
So it's great that smp_mb helped here but I suspect in fact we
need locking. Laurent?
--
MST
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