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

["Michael S. Tsirkin" <mst@redhat.com>]

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.


> > 
> > > 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.

> > 
> > > > > 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.

> > 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.



> > 
> > > > >
> > > > > >
> > > > > >
> > > > > >
> > > > > > > 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.

> 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.


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