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Subject: Re: [virtio-comment] Re: [virtio-dev] [PATCH 08/11] transport-pci: Introduce virtio extended capability
On Wed, Apr 12, 2023 at 12:20âPM Michael S. Tsirkin <mst@redhat.com> wrote: > > 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. > Ok. > > > 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. If we care about boot speed we should avoid using the PCI layer in the guest completely. Thanks > > > > WDYT? > > > > > > > > > -- > > > MST > > > > > > This publicly archived list offers a means to provide input to the > OASIS Virtual I/O Device (VIRTIO) TC. > > In order to verify user consent to the Feedback License terms and > to minimize spam in the list archive, subscription is required > before posting. > > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > List help: virtio-comment-help@lists.oasis-open.org > List archive: https://lists.oasis-open.org/archives/virtio-comment/ > Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf > List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists > Committee: https://www.oasis-open.org/committees/virtio/ > Join OASIS: https://www.oasis-open.org/join/ >
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