netdev - Re: virtio-net: why not always to set avail->flags to VRING_AVAIL_F_NO

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Message-ID: <876173uu8u.fsf@rustcorp.com.au>
Date:	Fri, 05 Jun 2015 05:22:01 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Linhaifeng <haifeng.lin@...wei.com>, netdev@...r.kernel.org
Cc:	virtualization@...ts.linux-foundation.org,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: virtio-net: why not always to set avail->flags to VRING_AVAIL_F_NO_INTERRUPT

Linhaifeng <haifeng.lin@...wei.com> writes:
> On 2015/6/4 9:13, Rusty Russell wrote:
>> Linhaifeng <haifeng.lin@...wei.com> writes:
>>> Hi,
>>>
>>> I'm a newbie and have a question about vring_new_virtqueue function.
>>>
>>> Why we set avail->flags to VRING_AVAIL_F_NO_INTERRUPT when no callbacks?
>>> I think we should set avail->flags to VRING_AVAIL_F_NO_INTERRUPT even if no callbacks.
>> 
>> Hi Linhaifeng,
>> 
>>         Not sure I understand your question, but I'll try to answer.
>> 
>> We don't set VRING_AVAIL_F_NO_INTERRUPT if there's a callback because we
>> want that callback called.  Otherwise callback will never be used.
>> 
>> Cheers,
>> Rusty.
>> 
>> 
>
> Hi Rusty,
>
> Thank you for your response.
>
> I mean should we set VRING_AVAIL_F_NO_INTERRUPT when virtqueue is initialized whether there is callback or not?
> As it would be set in function virtqueue_disable_cb and virtqueue_enable_cb_prepare later.

You're right, the callers could virtqueue_enable() specifically, but
I think most callers will want the virtqueue enabled immediately.

And remember, the VRING_AVAIL_F_NO_INTERRUPT is just a *hint*: the
device is allows to ignore it (this can happen due to race conditions,
for example).

Thanks,
Rusty.
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