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From: Laszlo Ersek <lersek@redhat.com>
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Date: Fri, 19 Feb 2016 21:57:18 +0100
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Subject: Re: [Qemu-devel] [PATCH 1/1] virtio-rng: fix condition checking on
 period_ms
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To: Wei Huang <wei@redhat.com>, qemu-devel@nongnu.org
Cc: qemu-trivial@nongnu.org, mst@redhat.com

On 02/19/16 19:13, Wei Huang wrote:
> The condition checking on vrng->conf.period_ms appears to be wrong,
> conflicting with the error comment following it.
> 
> Signed-off-by: Wei Huang <wei@redhat.com>
> ---
>  hw/virtio/virtio-rng.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio-rng.c b/hw/virtio/virtio-rng.c
> index 473c044..a06427c 100644
> --- a/hw/virtio/virtio-rng.c
> +++ b/hw/virtio/virtio-rng.c
> @@ -149,7 +149,7 @@ static void virtio_rng_device_realize(DeviceState *dev, Error **errp)
>      VirtIORNG *vrng = VIRTIO_RNG(dev);
>      Error *local_err = NULL;
>  
> -    if (!vrng->conf.period_ms > 0) {
> +    if (!(vrng->conf.period_ms > 0)) {
>          error_setg(errp, "'period' parameter expects a positive integer");
>          return;
>      }
> 

The current condition is absolutely weird, but I think it happens to
work correctly:

Period_ms has type uint32_t. If it is positive, then !period_ms is zero.
0>0 is false, hence the error message is not printed.

If period_ms is zero, then !period_ms is 1. 1>0 is true, hence the error
message is printed.

I would rewrite the check as

    if (vrng->conf.period_ms == 0) {
        error_setg(...)

Thanks
Laszlo