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Subject: Re: [virtio-dev] [PATCH 08/11] transport-pci: Introduce virtio extended capability
On Wed, Apr 12, 2023 at 12:21âPM Michael S. Tsirkin <mst@redhat.com> wrote: > > On Wed, Apr 12, 2023 at 12:04:45PM +0800, Jason Wang wrote: > > On Wed, Apr 12, 2023 at 3:01âAM Parav Pandit <parav@nvidia.com> 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? > > > > Right, but what I want to say is, such migration may cause unnecessary > > problems. And I don't see why it is a must for your legacy MMIO bar > > proposal. > > > > > > > > > " > > > > +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. > > > > What's the advantage of doing this? > > > > New drivers should provide backward compatibility so they must scan the pci cap. > > No, they can start with express cap. Finding one they can skip > the old cap completely. Then the driver can't work on the old device. Thanks > > > The Old driver can only scan the pci cap. > > > > > New fields donât need to be placed in PCI cap, because no driver is looking there. > > > > It would be much more simple if we forbid placing new fields in the > > PCI cap, it is already out of space. > > > > Thanks > > > > > > > > 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. > > > 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. >
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