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Subject: Re: [virtio-comment] [PATCH] virtio-net: support per-queue coalescing moderation
> On Wed, Feb 08, 2023 at 07:53:09PM +0200, Alvaro Karsz wrote: > > > > From: Michael S. Tsirkin <mst@redhat.com> > > > > Sent: Wednesday, February 8, 2023 9:48 AM > > > > > > > > On Wed, Feb 08, 2023 at 02:44:37PM +0000, Parav Pandit wrote: > > > > > > > > > > > From: Michael S. Tsirkin <mst@redhat.com> > > > > > > Sent: Wednesday, February 8, 2023 9:43 AM > > > > > > > > > > > > On Wed, Feb 08, 2023 at 02:37:55PM +0000, Parav Pandit wrote: > > > > > > > > > > > > > > > From: Michael S. Tsirkin <mst@redhat.com> > > > > > > > > Sent: Wednesday, February 8, 2023 9:18 AM > > > > > > > > > > > > > > > > On Wed, Feb 08, 2023 at 07:30:34PM +0800, Heng Qi wrote: > > > > > > > > > > I see two options. > > > > > > > > > > 1. Just have per VQ params. Software has the full knowledge > > > > > > > > > > of in which it is > > > > > > > > operating, and state remains at software level. > > > > > > > > > > This effectively achieves both the mode. > > > > > > > > > > > > > > > > > > > > 2. Have a mode cmd, > > > > > > > > > > Mode = (a) per device or (b) per VQ (c) disable After the > > > > > > > > > > mode is set, driver can set per device or per VQ. > > > > > > > > > > > > > > > > > > I find this more clear. > > > > > > > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > > > > > > > > > Rereading this I think I misunderstood the proposal. > > > > > > > > Now we are burning memory on maintaining mode, and this > > > > > > > > information is duplicated. > > > > > > > > > > > > > > > It is not maintained in the pci resident memory, so it doesn't hurt. > > > > > > > > > > > > > > > I'd say let's just add a new command COAL_QUEUE_SET with vqn as > > > > > > parameter. > > > > > > > > Existing commands are simply defined as a shortcut to running > > > > > > > > COAL_QUEUE_SET on all tx/rx queues respectively. > > > > > > > > > > > > > > > > Latest command dictates the parameters. To disable just set > > > > > > > > everything to 0 (btw we should make this explicit in the spec, > > > > > > > > but it can be > > > > > > guessed from: > > > > > > > > Upon reset, a device MUST initialize all coalescing parameters to 0. > > > > > > > > ) > > > > > > > > > > > > > > > Switching between the modes (per q vs per device) implicitly is > > > > > > > ambiguous > > > > > > and it only means device may need to iterate. > > > > > > > > > > > > hmm i feel it's only ambiguous because i failed to explain in well. > > > > > > > > > > > > > This state is either better maintained in sw by always having per > > > > > > > vq or have > > > > > > clearly defined mode of what device should do. > > > > > > > > > > > > > > Per Q is very common even for several years old devices. > > > > > > > Last time I counted, there were at least 15 such devices supporting it. > > > > > > > > > > > > > > So actual usage wise, I practically see that most implementations > > > > > > > will end up > > > > > > with per vq mode. > > > > > > > I like to hear from Heng or Alvaro if they see any use of per device. > > > > > > > > > > > > > > > > > > Right so given this, most devices will be in per queue mode all the > > > > > > time. why do you want a mode then? just keep per queue. > > > > > > existing commands are kept around for compat but internally just > > > > > > translate to per-queue. > > > > > Since the space is not released, do we need to keep the compat? > > > > > > > > It's been accepted for half a year so we can't say for sure no one built this. > > > That is likely but we should have the ability to have the Errata/ECN to correct it, specially for unrelease spec. > > > > > > > The way I propose is just a bit of firmware on device that scans all queues and > > > > copies same parameters everywhere. > > > This scanning loop in sw appears cheaper to me than some embedded fw. > > > But is not a lot of concern. > > > > > > > Seems easier than worrying about this, > > > > and we get disabling coalescing for free which you wanted. With an extra mode > > > > its extra logic in the device fast path. Maybe it's cheap on hardware side but in > > > > software it's an extra branch, not free. > > > > > > Most performant data path wouldn't implement and read the extra mode. > > > It is always fw that is going to program same value, or per queue valued or disable value in each Q regardless whichever way we craft the CVQ cmd. > > > > > > The sequence that bothers me is below. > > > 1. driver set global params > > > 2. few minutes later, now driver set param for Q=1 > > > > > > On this command, a device need to decide: > > > Should Q = 2 to N > > > (a) either work with previous globals, or > > > (b) because per Q was set for one queue, they rest of the queues implicitly disable it. > > > > > > If it is (b), > > > When a command on Q object =1 is issued, it affects other Q objects. <- This I want to avoid. > > > A cmd that modifies the object, should only modify that object. > > > > > > If it is (a), it is mixed mode operation, which is ambiguous definition. > > > > > > A better semantic is to define such change at device level and no extra cost in the data path. > > > > I think that (a) is the way to go. > > I don't think that we should work with operation modes at all. > > > > In my opinion: > > > > We should have 2 features: > > VIRTIO_NET_F_PERQUEUE_NOTF_COAL and VIRTIO_NET_F_NOTF_COAL. > > > > VIRTIO_NET_F_PERQUEUE_NOTF_COAL sets per queue parameters, and > > VIRTIO_NET_F_NOTF_COAL sets parameters for all queues. > > > > VIRTIO_NET_F_NOTF_COAL has 2 commands: > > VIRTIO_NET_CTRL_NOTF_COAL_RX_SET > > VIRTIO_NET_CTRL_NOTF_COAL_TX_SET > > > > VIRTIO_NET_F_PERQUEUE_NOTF_COAL has 2 commands: > > VIRTIO_NET_CTRL_NOTF_COAL_PER_QUEUE_TX_SET > > VIRTIO_NET_CTRL_NOTF_COAL_PER_QUEUE_RX_SET > > > > We can see VIRTIO_NET_CTRL_NOTF_COAL_RX_SET as a virtio level shortcut > > for setting all queues with one command, exactly as intended with > > rx_qid= 0xFFFF, and without breaking devices following the current > > spec. > > > > The device's FW can decide if it stores parameters received with > > VIRTIO_NET_CTRL_NOTF_COAL_RX_SET in a global set, or if it iterates > > through all queues, but IMO the best way it to iterate through all > > queues. > > > > Seems like a win-win situation to me. > > We achieve the same functionality as described in the patch, but > > without breaking devices following the current spec. > > > > Now, if we follow this method, > > VIRTIO_NET_CTRL_NOTF_COAL_PER_QUEUE_RX_SET with rx_qid= 0xFFF seems > > redundant. > > If VIRTIO_NET_F_PERQUEUE_NOTF_COAL requires VIRTIO_NET_F_NOTF_COAL, a > > device supporting VIRTIO_NET_F_PERQUEUE_NOTF_COAL can achieve the same > > functionality with the VIRTIO_NET_CTRL_NOTF_COAL_RX_SET command. > > Yes. Just some comments: > > - I don't think we need two commands. We have RX and TX because we > did not have vq number previously. No we do so just pass that. > It's also clearer since struct name can match command exactly. > I agree, good point. Just need to be sure that we won't put the control vq index in there :) (maybe we should mention this in the conformance sections?) > - Once we do that we can use a short _VQ_ instead of the wordy "PER_QUEUE". > > - Accordingly a well understood "vqn" instead of our own "qid" which > we then need to define. > You're right, I used the definitions from the proposed patch to make a point, I wasn't trying to imply that that's what it should be called. > - And yes no need for a reserved "qid" - it's a distinct command. > > -- > MST >
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