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Message-ID: <CACGkMEuhFjXCBpVVTr7fvu4ma1Lw=JJyoz8rACb_eqLrWJW6aw@mail.gmail.com> Date: Wed, 20 Jul 2022 15:42:19 +0800 From: Jason Wang <jasowang@...hat.com> To: "Michael S. Tsirkin" <mst@...hat.com> Cc: Alvaro Karsz <alvaro.karsz@...id-run.com>, Jakub Kicinski <kuba@...nel.org>, netdev <netdev@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com> Subject: Re: [PATCH net-next v4] net: virtio_net: notifications coalescing support On Wed, Jul 20, 2022 at 3:15 PM Michael S. Tsirkin <mst@...hat.com> wrote: > > On Wed, Jul 20, 2022 at 10:07:11AM +0300, Alvaro Karsz wrote: > > > Hmm. we currently (ab)use tx_max_coalesced_frames values 0 and 1 to mean tx > > napi on/off. > > > However I am not sure we should treat any value != 1 as napi on. > > > > > > I don't really have good ideas - I think abusing coalescing might > > > have been a mistake. But now that we are there, I feel we need > > > a way for userspace to at least be able to figure out whether > > > setting coalescing to 0 will have nasty side effects. > > > > > > So, how can I proceed from here? > > Maybe we don't need to use tx napi when this feature is negotiated (like Jakub > > suggested in prev. versions)? > > It makes sense, since the number of TX notifications can be reduced by setting > > tx_usecs/tx_max_packets with ethtool. > > > Hmm Jason had some ideas about extensions in mind when he > coded the current UAPI, let's see if he has ideas. > I'll ruminate on compatibility a bit too. I might be wrong, but using 1 to enable tx napi is a way to try to be compatible with the interrupt coalescing. That is, without notification coalescing, if 1 is set, we enable tx notifications (and NAPI) for each packet. This works as if tx-max-coalesced-frames is set to 1 when notification coalescing is negotiated. Thanks > > > > It's also a bit of a spec defect that it does not document corner cases > > > like what do 0 values do, are they different from 1? or what are max values. > > > Not too late to fix? > > > > > > I think that some of the corner cases can be understood from the coalescing > > values. > > For example: > > if rx_usecs=0 we should wait for 0 usecs, meaning that we should send a > > notification immediately. > > But if rx_usecs=1 we should wait for 1 usec. > > The case with max_packets is a little bit unclear for the values 0/1, and it > > seems that in both cases we should send a notification immediately after > > receiving/sending a packet. > > > > > > > So the spec says > > > Device supports notifications coalescing. > > > > > > which makes more sense - there's not a lot guest needs to do here. > > > > > > Noted. > > > > > parameters? > > > > > > I'll change it to "settings". > > > > > why with dash here? And why not just put the comments near the fields > > > themselves? > > > > > > Noted. >
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