Re: [virtio-comment] Re: [virtio-dev] Re: [virtio-comment] Re: [PATCH v18] virtio-net: support inner header hash

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

On Thu, Jun 22, 2023 at 08:27:48PM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Thursday, June 22, 2023 3:02 PM
> 
> 
> > > Existing fields used by existing drivers must be in MMIO.
> > > Device does not have a choice.
> > 
> > They can if they like mask some less important features reducing the footprint.
> How do to that? How would device know to not have existing fields on MMIO?

As you said yourself, feature bits also have cost. They must be
accessed with DMA too, not through MMIO.
This set of features can have more bits than the one in MMIO.


> > This will be the cost/benefit analysis each vendor does separately.
> > 
> > Case in point, if inner hash support is in 1.3, and linux drivers don't really use
> > that at all for a year or two because it's only used by alibaba in their stack, and
> > by the time linux starts using them it also supports the new commands, then
> > practically devices do not care and can just mask the feature bit in MMIO.
> > 
> Once it lands to config space, it becomes existing fields.

What's in which version of the spec is immaterial.  If all guests using
it also support config space through DMA then it does not matter that
technically there was a spec version which had inner hash but not DMA.

> More below.
> > 
> 
> > > > If they want to cut at 1.3 time, they can, but they also can cut it
> > > > at 1.2 time, this means some features will not be accessible through
> > > > MMIO only through the new commands.
> > > >
> > > New commands services new fields by newer driver.
> > > Newer driver can call new command to get new and old fields both.
> > 
> > yes.
> > 
> > > So 1.3 and 1.4 devices cannot optimize for older drivers.
> > 
> > 
> > I don't know what this means.
> > 
> I guess next below question answers it.

I still don't know what did you mean by "So 1.3 and 1.4 devices cannot
optimize for older drivers". If it's immaterial, fine.

-- 
MST