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Subject: Re: [virtio-comment] Re: [PATCH 1/2] virtio: introduce selective queue enabling
On Thu, 8 Jun 2023 16:18:23 +0200, Eugenio Perez Martin <eperezma@redhat.com> wrote: > On Thu, Jun 8, 2023 at 2:24âPM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote: > > > > On Tue, 6 Jun 2023 15:19:53 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > On Tue, Jun 06, 2023 at 06:04:32PM +0000, Parav Pandit wrote: > > > > Hi Eugenio, > > > > > > > > > From: Eugenio Péez <eperezma@redhat.com> > > > > > Sent: Tuesday, June 6, 2023 1:55 PM > > > > > > > > > > This patch allows the driver to start the device (as set DRIVER_OK) with only > > > > > some queues enabled, and then enable another queues later. > > > > > > > > > > This is the current way to migrate net device state through control virtqueue, in > > > > > a software assisted framework with vDPA: > > > > > * First, only net CVQ is enabled at DRIVER_OK > > > > > * All the control commands (mac address, mq, etc) needed for the device to > > > > > behave the same as the source of migration are sent > > > > > * Finally all the dataplane queues are enabled. > > > > > > > > > > Signed-off-by: Eugenio Péez <eperezma@redhat.com> > > > > > --- > > > > > content.tex | 15 +++++++++++++-- > > > > > 1 file changed, 13 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/content.tex b/content.tex > > > > > index d2ab9eb..ea4a6d8 100644 > > > > > --- a/content.tex > > > > > +++ b/content.tex > > > > > @@ -99,10 +99,10 @@ \section{Feature Bits}\label{sec:Basic Facilities of a > > > > > Virtio Device / Feature B \begin{description} > > > > > \item[0 to 23, and 50 to 127] Feature bits for the specific device type > > > > > > > > > > -\item[24 to 41] Feature bits reserved for extensions to the queue and > > > > > +\item[24 to 42] Feature bits reserved for extensions to the queue and > > > > > feature negotiation mechanisms > > > > > > > > > > -\item[42 to 49, and 128 and above] Feature bits reserved for future extensions. > > > > > +\item[43 to 49, and 128 and above] Feature bits reserved for future > > > > > extensions. > > > > > \end{description} > > > > > > > > > > \begin{note} > > > > > @@ -398,6 +398,13 @@ \section{Virtqueues}\label{sec:Basic Facilities of a > > > > > Virtio Device / Virtqueues} Every driver and device supports either the Packed > > > > > or the Split Virtqueue format, or both. > > > > > > > > > > +\subsection{Virtqueue selective enabling}\label{sec:Basic Facilities of > > > > > +a Virtio Device / Virtqueues / Selective Virtqueue Enable} > > > > > + > > > > > +If the driver negotiates VIRTIO\_F\_RING\_ENABLE\_ANYTIME, it can > > > > > +enable individual virtqueues both before and after the DRIVER\_OK. The > > > > > +way to enable a virtqueue is transport specific, but it mimics the > > > > > +re-enabling after an hypothetical reset. > > > > > + > > > > > \subsection{Virtqueue Reset}\label{sec:Basic Facilities of a Virtio Device / > > > > > Virtqueues / Virtqueue Reset} > > > > > > > > > > When VIRTIO_F_RING_RESET is negotiated, the driver can reset a virtqueue > > > > > @@ -870,6 +877,10 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved > > > > > Feature Bits} > > > > > \ref{devicenormative:Basic Facilities of a Virtio Device / Feature Bits} for > > > > > handling features reserved for future use. > > > > > > > > > > + \item[VIRTIO_F_RING_ENABLE_ANYTIME(42)] This feature indicates that > > > > > + the driver can enable a queue both before and after DRIVER\_OK. > > > > > + See \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Selective > > > > > Virtqueue Enable}. > > > > > + > > > > > > > > Can you please highlight, current spec normative which indicates that > > > > all queues must be enabled before DRIVER_OK and more queues cannot be > > > > enabled after DRIVER_OK? > > > > > > > > > This one: > > > \drivernormative{\subsection}{Device Initialization}{General Initialization And Device Operation / Device Initialization} > > > > > > it does not say "all queues" if you want to split hairs but I think > > > assuming it's safe to enable queues later is dangerous. > > > > Although there is no "all queues" here, if the queue is not ready, the device > > may use all the queues after "Device OK". So I also think it might be dangerous. > > > > But I also ran into this problem. We want the device to support 100 queues, but > > we also want to use only 16 queues by default. The user can then manually scale > > up to 100, if the user has a large memory. > > > > So I think, the point is that "num_queues" is the max and default value. Can we > > change "num_queues" to default, then we introduced a new value as max. > > > > Does this way solve your problem? @Eugenio > > > > It would need a tweak, but it gives me an idea. > > I think you're talking about what the device offers, isn't it? Yes, I want the driver offers a default-num-queues and a max-num-queues. Thanks > I would > need to modify it from the driver. > > But it has an interesting question, how to allocate "less vrings" than > virtio_pci_common_cfg.num_queues? I guess it is ok to set all vq > addresses to a vring that never increments avail_idx? The driver still > needs to allocate memory for that one and lose start time configuring > them. > > Maybe for net is valid to leave them unconfigured? it should only use > one vq pair and CVQ by default. > > My proposal could be done at the virtio-net level, allowing > max_virtqueue_pairs to be 0 from the beginning. For example, allowing > the driver to write it before DRIVER_OK. In that case, no data vq is > enabled and only CVQ process requests after DRIVER_OK. After all the > config is written, the driver changes the number of active queues with > VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET. > > I think it is better solved in a generic way, but I'm ok with this > -net specific too. > > Thanks! > > > Thanks. > > > > > > > > > > > > E.g. vhost user backends tend to assume no new queues appear > > > after DRIVER_OK. > > > > > > -- > > > 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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