On Wed, Apr 12, 2023 at 12:07:26PM +0800, Jason Wang wrote: > On Wed, Apr 12, 2023 at 5:25 AM Michael S. Tsirkin <[email protected]> wrote: > > > > On Tue, Apr 11, 2023 at 07:01:16PM +0000, Parav Pandit wrote: > > > > > > > From: [email protected] <[email protected]> > > > > 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 don’t 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 > > > doesn’t 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 > > --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
