netdev - Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi

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Date:   Tue, 29 Aug 2017 18:55:32 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Koichiro Den <den@...ipeden.com>, Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit
 path if no tx napi

On Tue, Aug 29, 2017 at 4:40 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Tue, Aug 29, 2017 at 03:53:08PM -0400, Willem de Bruijn wrote:
>> On Tue, Aug 29, 2017 at 3:42 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
>> > On Tue, Aug 29, 2017 at 03:35:38PM -0400, Willem de Bruijn wrote:
>> >> By the way, I have had an unrelated patch outstanding for a while
>> >> to have virtio-net support the VIRTIO_CONFIG_S_NEEDS_RESET
>> >> command. Will send that as RFC.
>> >
>> > Oh nice.
>>
>> Great :)
>>
>> > One needs to be careful about locking there which is why
>> > no devices support that yet.
>>
>> I originally wrote it based on the virtnet_reset function introduced
>> for xdp. Calling this from virtnet_config_changed_work is non trivial,
>> as virtnet_freeze_down waits until no config worker is running.
>>
>> Otherwise, I could not find any constraints on when freeze may be
>> called, and it largely follows the same path. I hope I didn't miss anything.
>
> The issue is that on freeze processes are not running so we
> generally know no new packets will arrive (might be wrong
> for bridging, then it's a bug). On device error you must
> prevent new skbs from coming in, etc.

Thanks a lot for the quick review. I had indeed not yet figured out
which invariants freeze can depend on that are not universal. Same
for the virtnet_reset call from the .ndo_xdp. Will need to take a much
better look at that.