RE: [virtio-comment] Re: [PATCH v6 2/5] virtio-net: Add flow filter capabilities read commands

[Parav Pandit <parav@nvidia.com>]

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Monday, November 27, 2023 5:17 PM
> 
> On Mon, Nov 27, 2023 at 11:40:25AM +0000, Parav Pandit wrote:
> >
> > > From: Michael S. Tsirkin <mst@redhat.com>
> > > Sent: Monday, November 27, 2023 4:58 PM
> > >
> > > On Mon, Nov 27, 2023 at 10:19:49AM +0000, Parav Pandit wrote:
> > > >
> > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > Sent: Friday, November 24, 2023 5:04 PM
> > > > >
> > > > > On Fri, Nov 24, 2023 at 06:31:03AM +0000, Parav Pandit wrote:
> > > > > >
> > > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > Sent: Friday, November 24, 2023 11:48 AM
> > > > > > >
> > > > > > > On Fri, Nov 24, 2023 at 05:40:26AM +0000, Parav Pandit wrote:
> > > > > > > > > we
> > > > > > > > > strongly suggest that *drivers* support both old and new
> > > > > > > > > mechanism, and then *devices* will only implement what's
> > > required.
> > > > > > > > There are other examples in the same document that makes
> > > > > > > > things worst
> > > > > > > with old and new.
> > > > > > > >
> > > > > > > > Also there is literally no way to enforce that driver
> > > > > > > > supports both and new. It is just sounds like an excuse to
> > > > > > > > force infinite config space.
> > > > > > >
> > > > > > > There is a very simple method though.  We allow devices to
> > > > > > > expose a subset of features when DMA is not used. So drivers
> > > > > > > that want maximum features will always opt for DMA. We can
> > > > > > > also strongly recommend that all drivers support DMA if available.
> > > > > > Yeah, don't see how this is elegant at all with all mixed bits.
> > > > >
> > > > > It's elegant because simple low end devices can cheaply
> > > > > implement MMIO and not worry about DMA.
> > > > >
> > > > It is not of much help in this case because any low end cheap
> > > > device which
> > > want to support flow filter commands need to have CVQ anyway.
> > > > And hence reusing the same CVQ is more elegant that already does the
> DMA.
> > > >
> > > > So CVQ is fulfilling all the below needs.
> > > > 1. Single interface for the get/set config flow filters 2. DMA the
> > > > data 3. Not have any partial issues
> > >
> > > I don't know what these are.
> > I mean partial writes for fields.
> 
> We don't allow these and in any case, writeble fields are best avoided.
> 
Right. This is why flow filter objects are created using cvq, and its associated caps also comes via same cvq channel.

> 
> > >
> > > > 4. provides consistent structures that provisioning side will be
> > > > able to use
> > >
> > > Problem for provisioning is extra definitions will be needed, in a
> > > device specific way.
> > In vdpa tool and other OS tools of iproute2 developed, setting and getting
> those device specific values are useful.
> > It is ok.
> 
> It does not become ok just by saying so. You are taking a single RO value and
> instead of it having an address there are now 2 other ways to address it. And
> you fail to see the problem and the pain you are inflicting on software
> developers. Just stick with an address if you can.
There is zero problem with sw.
Sw just need to issue send_command() and done with it, like rest of the commands.
A pain would be create yet another DMA interface.

> 
> > >
> > > > > > Nor do I see any enforcement, single method via cvq still holds strong.
> > > > >
> > > > > You don't need to enforce things, if people want to put a lot of
> > > > > RAM on device and put it in a register let them.
> > > > >
> > > > Not enforced. It uses the CVQ for flow group and flow filter life
> > > > cycles and for
> > > the sharing this config as well.
> > > > Also aligns with stats that rest also agreed on.
> > >
> > > I am talking about your attempt to generally say "no more config
> > > fields everything must be in CVQ".
> > Config fields for initialization time is fine as the spec allows it today.
> > Things which can differ, it is ok to use cvq interface.
> 
> I don't know what does "Things that can differ" means. Generally device caps
> are perfect for config space. Accessed at init time only, RO.
> 
You ignore the comment I answered before that proposal here is not based on RO/RW.
It is based on initialization time vs run time.

> > > I think it's wrong definitiely for non network devices must
> > > sometimes for network too and generally we need a solution for
> > > config over DMA. This specific thing - whether it fits in CVQ is a
> > > separate discussion.
> > >
> > I explained it before, that 6 out of 19 devices has cvq which are complex
> enough doing things over cvq.
> > These are non-network devices already.
> >
> > If one of those remaining device becomes complex, it is likely it will need a
> cvq to suffice for the dma interface and it can just do with depth = 1.
> 
> Using generic caps and not net specific ones is a good idea.
> 
context here is cvq and net.

> 
> 
> > >
> > > > > >
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > > The method proposed here is elegant and clearly promote
> > > > > > > > one way to do
> > > > > > > things for driver and device with predictability.
> > > > > > > >
> > > > > > >
> > > > > > > I don't see it as elegant at all. What is elegant is *a
> > > > > > > single
> > > > > > > tag* that describes each property of the device. And this
> > > > > > > single tag should be
> > > > > good for everything:
> > > > > > > driver, provisioning, migration. And config space offset serves as
> such.
> > > > > > The single tag is the set of structures.
> > > > >
> > > > > I have no idea how this will work. If migration format i started
> > > > > reviewing is anything to go by then there will be a huge
> > > > > elaborate structure nothing single or simple. By comparison
> > > > > there's already a proposal how provisioning can work by supplying
> config space.
> > > > > it is just a clean model to grasp.
> > > > >
> > > > The provisioning model is simple is to supply all the configuration.
> > > > To draw parallels to some sw side,
> > > >
> > > > There is per functionality socket option to set things, instead of
> > > > one giant
> > > structure.
> > > > There is per functionality ethtool option/cmd instead of Set
> > > > ALL/get ALL
> > > enforcement.
> > >
> > > I'm not sure how much of a parallel one can draw.
> > > Do not see a lot of similarity.
> > For lot of configuration they are similar that happens at slow pace.
> >
> > > Devices commonly use register map. Everyone understands this paradigm.
> > >
> > For initialization early device setup time, yes.
> >
> > >
> > > I am not altogether happy with the way you are making migration
> > > generate duplicate definitions for lots of things we already have definitions
> for.
> > > Having a 3rd one for provisioning? Gimme a break.
> >
> > For migration, we are not duplicating. Some structures are not well defined,
> it has some duplication.
> 
> And fyi it's already making people unhappy.
> 
Those exceptions are not the interesting one to take as example here.

> > But large part seems be able to utilized pre-defined structs.
> > And here for flow filter also same structs will be used.
> 
> So if there's a 64 bit bitmap in config space, then provisioning command which
> already gets config space can just use its offset.
> Simpler, better.
> 
It is not simple to implement per device unique config space as we discussed already.
And no need another DMA interface either as cvq service that need already.

> 
> > >
> > >
> > > > > > Provisioning access them via owner device.
> > > > > > Member driver access them via CVQ or 1.2 legacy config space.
> > > > >