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Message-ID: <0dd338bb-0fbe-b9d5-0962-d47ac2de4c4e@redhat.com>
Date:   Thu, 23 Sep 2021 08:26:06 +0200
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Subject: Re: [PATCH 1/4] hwrng: virtio - add an internal buffer
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To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Alexander Potapenko <glider@google.com>,
        linux-crypto@vger.kernel.org, Dmitriy Vyukov <dvyukov@google.com>,
        rusty@rustcorp.com.au, amit@kernel.org, akong@redhat.com,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Matt Mackall <mpm@selenic.com>,
        virtualization@lists.linux-foundation.org
References: <20210922170903.577801-1-lvivier@redhat.com>
 <20210922170903.577801-2-lvivier@redhat.com>
 <20210922145651-mutt-send-email-mst@kernel.org>
From:   Laurent Vivier <lvivier@redhat.com>
In-Reply-To: <20210922145651-mutt-send-email-mst@kernel.org>
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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.

> 
>>   
>>   	/* 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