Re: [virtio-dev] [PATCH 08/11] transport-pci: Introduce virtio extended capability

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

On Wed, Apr 12, 2023 at 12:07:26PM +0800, Jason Wang wrote:
> On Wed, Apr 12, 2023 at 5:25âAM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Apr 11, 2023 at 07:01:16PM +0000, Parav Pandit wrote:
> > >
> > > > From: virtio-dev@lists.oasis-open.org <virtio-dev@lists.oasis-open.org> On
> > > > Behalf Of Jason Wang
> > > > Sent: Monday, April 10, 2023 11:29 PM
> > >
> > > > > However, it is not backward compatible, if the device place them in
> > > > > extended capability, it will not work.
> > > > >
> > > >
> > > > It is kind of intended since it is only used for new PCI-E features:
> > > >
> > > New fields in new extended pci cap area is fine.
> > > Migrating old fields to be present in the new extended pci cap, is not your intention. Right?
> > >
> > > > "
> > > > +The location of the virtio structures that depend on the PCI Express
> > > > +capability are specified using a vendor-specific extended capabilities
> > > > +on the extended capabilities list in PCI Express extended configuration
> > > > +space of the device.
> > > > "
> > > >
> > > > > To make it backward compatible, a device needs to expose existing
> > > > > structure in legacy area. And extended structure for same capability
> > > > > in extended pci capability region.
> > > > >
> > > > > In other words, it will have to be a both places.
> > > >
> > > > Then we will run out of config space again?
> > > No.
> > > Only currently defined caps to be placed in two places.
> > > New fields donât need to be placed in PCI cap, because no driver is looking there.
> > >
> > > We probably already discussed this in previous email by now.
> > >
> > > > Otherwise we need to deal with the
> > > > case when existing structures were only placed at extended capability. Michael
> > > > suggest to add a new feature, but the driver may not negotiate the feature
> > > > which requires more thought.
> > > >
> > > Not sure I understand feature bit.
> >
> > This is because we have a concept of dependency between
> > features but not a concept of dependency of feature on
> > capability.
> >
> > > PCI transport fields existence is usually not dependent on upper layer protocol.
> > >
> > > > > We may need it even sooner than this because the AQ patch is expanding
> > > > > the structure located in legacy area.
> > > >
> > > > Just to make sure I understand this, assuming we have adminq, any reason a
> > > > dedicated pcie ext cap is required?
> > > >
> > > No. it was my short sight. I responded right after above text that AQ doesnât need cap extension.
> >
> >
> >
> > You know, thinking about this, I begin to feel that we should
> > require that if at least one extended config exists then
> > all caps present in the regular config are *also*
> > mirrored in the extended config. IOW extended >= regular.
> > The reason is that extended config can be emulated more efficiently
> > (2x less exits).
> 
> Any reason for it to get less exits?

For a variety of reasons having to do with buggy hardware e.g. linux
likes to use cf8/cfc for legacy ranges. 2 accesses are required for each
read/write.  extended space is just 1.


> At least it has not been done in
> current Qemu's emulation. (And do we really care about the performance
> of config space access?)
> 
> Thanks

For boot speed, yes. Not minor 5% things but 2x, sure.

> > WDYT?
> >
> >
> > --
> > MST
> >