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

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

On Mon, Apr 10, 2023 at 03:16:46PM +0800, Jason Wang wrote:
> On Mon, Apr 10, 2023 at 2:24âPM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Mon, Apr 10, 2023 at 09:36:17AM +0800, Jason Wang wrote:
> > > On Fri, Mar 31, 2023 at 7:00âAM Parav Pandit <parav@nvidia.com> wrote:
> > > >
> > > > PCI device configuration space for capabilities is limited to only 192
> > > > bytes shared by many PCI capabilities of generic PCI device and virtio
> > > > specific.
> > > >
> > > > Hence, introduce virtio extended capability that uses PCI Express
> > > > extended capability.
> > > > Subsequent patch uses this virtio extended capability.
> > > >
> > > > Co-developed-by: Satananda Burla <sburla@marvell.com>
> > > > Signed-off-by: Parav Pandit <parav@nvidia.com>
> > >
> > > Can you explain the differences compared to what I've used to propose?
> > >
> > > https://www.mail-archive.com/virtio-dev@lists.oasis-open.org/msg08078.html
> > >
> > > This can save time for everybody.
> > >
> > > Thanks
> >
> > BTW another advantage of extended capabilities is - these are actually
> > cheaper to access from a VM than classic config space.
> 
> Config space/BAR is allowed by both of the proposals or anything I missed?
> 
> >
> >
> > Several points
> > - I don't like it that yours is 32 bit. We do not need 2 variants just
> >   make it all 64 bit
> 
> That's fine.
> 
> > - We need to document that if driver does not scan extended capbilities it will not find them.
> 
> This is implicit since I remember we don't have such documentation for
> pci capability, anything makes pcie special?

yes - the fact that there are tons of existing drivers expecting
everything in standard space.


> >   And existing drivers do not scan them. So what is safe
> >   to put there? vendor specific? extra access types?
> 
> For PASID at least, since it's a PCI-E feature, vendor specific should
> be fine. Not sure about legacy MMIO then.
> 
> >   Can we make scanning these mandatory in future drivers? future devices?
> >   I guess we can add a feature bit to flag that.
> 
> For PASID, it doesn't need this, otherwise we may duplicate transport
> specific features.

i don't get it. what does PASID have to do with it?
A new feature will allow clean split at least:
we make any new features and new devices that expect
express capability depend on this new feature bit.

> >   Is accessing these possible from bios?
> 
> Not at least for the two use cases now PASID or legacy MMIO.

can't parse english here. what does this mean?


> >
> > So I like this one better as a basis - care reviewing it and adding
> > stuff?
> 
> There are very few differences and I will have a look.
> 
> Thanks
> 
> >
> > --
> > MST
> >