netdev - Re: [PATCH bpf-next v2 06/10] xsk: propagate napi

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Date:   Mon, 16 Nov 2020 13:01:40 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        magnus.karlsson@...el.com, ast@...nel.org, daniel@...earbox.net,
        maciej.fijalkowski@...el.com, sridhar.samudrala@...el.com,
        jesse.brandeburg@...el.com, qi.z.zhang@...el.com, kuba@...nel.org,
        edumazet@...gle.com, jonathan.lemon@...il.com, maximmi@...dia.com,
        intel-wired-lan@...ts.osuosl.org, netanel@...zon.com,
        akiyano@...zon.com, michael.chan@...adcom.com,
        sgoutham@...vell.com, ioana.ciornei@....com,
        ruxandra.radulescu@....com, thomas.petazzoni@...tlin.com,
        mcroce@...rosoft.com, saeedm@...dia.com, tariqt@...dia.com,
        aelior@...vell.com, ecree@...arflare.com,
        ilias.apalodimas@...aro.org, grygorii.strashko@...com,
        sthemmin@...rosoft.com, kda@...ux-powerpc.org
Subject: Re: [PATCH bpf-next v2 06/10] xsk: propagate napi_id to XDP socket Rx
 path


On 2020-11-16 12:55, Michael S. Tsirkin wrote:
> On Mon, Nov 16, 2020 at 12:04:12PM +0100, BjÃ¶rn TÃ¶pel wrote:
>> From: BjÃ¶rn TÃ¶pel <bjorn.topel@...el.com>
>>
>> Add napi_id to the xdp_rxq_info structure, and make sure the XDP
>> socket pick up the napi_id in the Rx path. The napi_id is used to find
>> the corresponding NAPI structure for socket busy polling.
>>
>> Acked-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
>> Signed-off-by: BjÃ¶rn TÃ¶pel <bjorn.topel@...el.com>
> 
> A bunch of drivers just pass in 0. could you explain when
> is that ok? how bad is it if the wrong id is used?
>

If zero is passed, which is a non-valid NAPI_ID, busy-polling will never
be performed.

Depending on the structure of the driver, napi might or might not be
initialized (napi_id != 0) or even available. When it wasn't obvious, I
simply set it to zero.

So, short; No harm if zero, but busy-polling cannot be used in an XDP
context.


[...]
>>   
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 21b71148c532..d71fe41595b7 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -1485,7 +1485,7 @@ static int virtnet_open(struct net_device *dev)
>>   			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
>>   				schedule_delayed_work(&vi->refill, 0);
>>   
>> -		err = xdp_rxq_info_reg(&vi->rq[i].xdp_rxq, dev, i);
>> +		err = xdp_rxq_info_reg(&vi->rq[i].xdp_rxq, dev, i, 0);
>>   		if (err < 0)
>>   			return err;
>>   
> 
> Should this be rq.napi.napi_id ?
>

Yes, if rq.napi.napi_id is valid here! Is it?


Cheers,
Björn