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Subject: Re: [virtio-comment] Re: [PATCH 0/2] Selective queue enabling
On Sun, Jun 25, 2023 at 01:31:36PM +0800, Jason Wang wrote: > On Sun, Jun 25, 2023 at 2:41âAM Eugenio Perez Martin > <eperezma@redhat.com> wrote: > > > > On Sun, Jun 18, 2023 at 1:08âAM Michael S. Tsirkin <mst@redhat.com> wrote: > > > > > > On Sat, Jun 17, 2023 at 02:53:46PM +0200, Eugenio Perez Martin wrote: > > > > On Fri, Jun 16, 2023 at 4:40âPM Michael S. Tsirkin <mst@redhat.com> wrote: > > > > > > > > > > On Thu, Jun 15, 2023 at 10:35:09AM +0200, Eugenio Perez Martin wrote: > > > > > > On Tue, Jun 13, 2023 at 2:29âPM Michael S. Tsirkin <mst@redhat.com> wrote: > > > > > > > > > > > > > > On Tue, Jun 13, 2023 at 12:12:43PM +0200, Eugenio Perez Martin wrote: > > > > > > > > On Tue, Jun 13, 2023 at 9:53âAM Michael S. Tsirkin <mst@redhat.com> wrote: > > > > > > > > > > > > > > > > > > On Tue, Jun 13, 2023 at 03:46:35AM -0400, Michael S. Tsirkin wrote: > > > > > > > > > > On Mon, Jun 12, 2023 at 09:56:57AM +0200, Eugenio Perez Martin wrote: > > > > > > > > > > > > > Another drawback of that is the long time a device can spend to > > > > > > > > > > > > > reconfigure its memory maps. > > > > > > > > > > > > > > > > > > > > > > > > The map is already used for cvq though, I don't see why it would take > > > > > > > > > > > > much more with this. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > In the case of vDPA HW, CVQ maps may not need to reach the device's or > > > > > > > > > > > platform IOMMU, so the modification of that mapping is way cheaper. > > > > > > > > > > > > > > > > > > > > Oh, I get it. It's setup time, but you are right. > > > > > > > > > > > > > > > > > > > > > > > taking that into account, does it make > > > > > > > > > > > > > sense to send a new version with the bit 1 on enable? > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks! > > > > > > > > > > > > > > > > > > > > > > > > Current hardware won't support the extra bit though. > > > > > > > > > > > > > > > > > > > > > > > > > > > Thinking more about this, I think the following works: > > > > > > > > > > > > > > > > > > - enable all queues before DRIVER_OK > > > > > > > > > - reset all data queues before DRIVER_OK > > > > > > > > > - DRIVER_OK > > > > > > > > > - CVQ commands > > > > > > > > > - re-enable data queues > > > > > > > > > > > > > > > > > > > > > > > > > > > No? > > > > > > > > > > > > > > > > > > > > > > > > > If it is valid to write to queue_reset before DRIVER_OK, then sure. I > > > > > > > > asked for that in the past at > > > > > > > > https://lists.oasis-open.org/archives/virtio-dev/202305/msg00329.html > > > > > > > > . > > > > > > > > > > > > > > Well then Jason said: > > > > > > > we should make svq work without RING_RESET > > > > > > > but now apparently we went full circle and propose > > > > > > > a feature bit after all? > > > > > > > > > > > > > > > > > > > My understanding is that RING_RESET covers more than just delaying the > > > > > > enabling of net data vqs, like to change the address or properties of > > > > > > vqs after DRIVER_OK. While the late enabling may be easy to implement > > > > > > in current hw, vq property changes may be more challenging. There are > > > > > > current HW that only support late enabling but not reconfigure the > > > > > > queues. Ideally, just the subset of late enabling is needed. > > > > > > > > > > > > Does it make sense to change the proposal to something in the line of: > > > > > > RING_F_LATE_ENABLING: This feature flag is a subset of _F_RING_RESET. > > > > > > If this feature is negotiated, all vqs start as if the vqs has been > > > > > > reset before DRIVER_OK. This feature does not allow resetting a queue > > > > > > once it is live. > > > > > > > > > > > > Thanks! > > > > > > > > > > Maybe. No idea - I am not a hardware designer. Can some hardware > > > > > vendors chime in and confirm? > > > > > It seems like a very very narrow "make my hack work" use-case. > > > > > And I'm not even sure how well it works. > > > > > For example what if cvq command enables rss? If the queues in > > > > > question are not enabled packets will be dropped. > > > > > So - fix some setups, break others? > > > > > > > > > > > > > I don't think it breaks, they are already dropped before setting > > > > DRIVER_OK anyway. If we restore the configuration before DRIVER_OK in > > > > the future, the device is supposed to drop packets meanwhile anyway, > > > > isn't it? > > > > > > > > > I just thought of an idea. Devices generally don't poke at rings > > > by themselves. How about at init time we cause kicks on data queues > > > to vmexit? Then on 1st kick we map hardware so following ones will > > > go straight to hardware. This will make sure cvq is processed > > > first. Hmm? > > This is not the case at least for vhost-net. It doesn't need to be > kicked in order to work. And it's an example that we need clearly > define the behaviour instead of depending on any existing > implementation details. This kind of guarantee however sounds generally useful to control which queues are accessed when. This is only needed after restore, which is ATM a vdpa thing, so can just be a vdpa flag, but I'm ok with it as a virtio flag too. > > > > > > > That would be possible, but nothing prevents the hw to start > > processing requests even without that first kick, isn't it? > > Yes. > > Thanks > > > > > I think that was discussed also in the past, as previous qemu patches > > gave a spurious kick to start queues just in case. I think the > > conclusion was that at least the vDPA device will start processing > > requests at DRIVER_OK, no need to wait for a kick. > > > > https://lore.kernel.org/all/CACGkMEt8cMM1UxVzxb0eHeaWSpR0ApvGzaF901vrM4m-uGMiPA@mail.gmail.com/ > > > > Thanks! > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > 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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