Re: [virtio-dev] [PATCH] virtio-net: Fix and update VIRTIO_NET_F_NOTF_COAL feature

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Feb 06, 2023 at 09:53:18PM +0000, Parav Pandit wrote:
> 
> 
> > From: virtio-dev@lists.oasis-open.org <virtio-dev@lists.oasis-open.org> On
> > Behalf Of Michael S. Tsirkin
> > 
> > On Mon, Feb 06, 2023 at 07:13:43PM +0000, Parav Pandit wrote:
> > >
> > >
> > > > From: virtio-dev@lists.oasis-open.org
> > > > <virtio-dev@lists.oasis-open.org> On Behalf Of Alvaro Karsz
> > > >
> > > > This patch makes several improvements to the notification coalescing
> > > > feature,
> > > > including:
> > > >
> > > > - Consolidating virtio_net_ctrl_coal_tx and virtio_net_ctrl_coal_rx
> > > >   into a single struct, virtio_net_ctrl_coal, as they are identical.
> > > > - Emphasizing that the coalescing commands are best-effort.
> > > > - Defining the behavior of coalescing with regards to delivering
> > > >   notifications when a change occur.
> > > >
> > > Patch needs to do one thing at a time.
> > > Please split above into three patches.
> > >
> > > > Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
> > > > ---
> > > >  device-types/net/description.tex | 40
> > > > ++++++++++++++++++--------------
> > > >  1 file changed, 22 insertions(+), 18 deletions(-)
> > > >
> > > > diff --git a/device-types/net/description.tex
> > > > b/device-types/net/description.tex
> > > > index 1741c79..2a98411 100644
> > > > --- a/device-types/net/description.tex
> > > > +++ b/device-types/net/description.tex
> > > > @@ -1514,15 +1514,12 @@ \subsubsection{Control
> > > > Virtqueue}\label{sec:Device Types / Network Device / Devi  If the
> > > > VIRTIO_NET_F_NOTF_COAL feature is negotiated, the driver can  send
> > > > control commands for dynamically changing the coalescing parameters.
> > > >
> > > > -\begin{lstlisting}
> > > > -struct virtio_net_ctrl_coal_rx {
> > > > -    le32 rx_max_packets;
> > > > -    le32 rx_usecs;
> > > > -};
> > > > +Note: In general, these commands are best-effort: A device could
> > > > +send a
> > > > notification even if it is not supposed to.
> > > >
> > > Please remove this note from this patch.
> > > Instead of Note, we need to describe this device requirements description.
> > > We better need to describe the motivation for it.
> > > We may want to say there may be jitter in notification, but device should not
> > be sending when it is not supposed to.
> > 
> > It's explicitly allowed:
> > 
> > split-ring.tex:The driver MUST handle spurious notifications from the device.
> > split-ring.tex:The device MUST handle spurious notifications from the driver.
> > 
> The intent is the guide to device implementation to have less spurious interrupts.
> Best effort wording says that device is free to implement timer of any granularity of choice, which kind of defeats the purpose of IM.
> 
> So, both can handle/generate spurious notifications, but it shouldn't be best line guidance.

Don't really see why not. Lots of spurious interrupts would defeat the
purpose of the feature clearly. But it might be handy e.g. for migration
purposes. Overall it's tough to document in detail but if there are
too many interrupts then surely it's a quality of implementation issue.
I feel trying to go into too much detail here will just pointlessly make
it verbose without making it clearer.

> > 
> > 
> > > I also have more description to add in this area with regards to GSO and LRO.
> > > I make humble suggestion that we draft is jointly in separate patch combining
> > these clarifications.
> > >
> > > > -struct virtio_net_ctrl_coal_tx {
> > > > -    le32 tx_max_packets;
> > > > -    le32 tx_usecs;
> > > > +\begin{lstlisting}
> > > > +struct virtio_net_ctrl_coal {
> > > > +    le32 max_packets;
> > > > +    le32 usecs;
> > > >  };
> > > >
> > > This is one good change to go as separate patch.
> > >
> > > >  #define VIRTIO_NET_CTRL_NOTF_COAL 6 @@ -1532,25 +1529,25 @@
> > > > \subsubsection{Control Virtqueue}\label{sec:Device Types / Network
> > > > Device / Devi
> > > >
> > > >  Coalescing parameters:
> > > >  \begin{itemize}
> > > > -\item \field{rx_usecs}: Maximum number of usecs to delay a RX
> > notification.
> > > > -\item \field{tx_usecs}: Maximum number of usecs to delay a TX
> > notification.
> > > > -\item \field{rx_max_packets}: Maximum number of packets to receive
> > > > before a RX notification.
> > > > -\item \field{tx_max_packets}: Maximum number of packets to send
> > > > before a TX notification.
> > > > +\item \field{usecs} for RX: Maximum number of usecs to delay a RX
> > > > notification.
> > > > +\item \field{usecs} for TX: Maximum number of usecs to delay a TX
> > > > notification.
> > > > +\item \field{max_packets} for RX: Maximum number of packets to
> > > > +receive
> > > > before a RX notification.
> > > > +\item \field{max_packets} for TX: Maximum number of packets to send
> > > > +before
> > > > a TX notification.
> > > >  \end{itemize}
> > > >
> > > s/for Rx/For receive virtqueue
> > > s/for Tx/For transmit virtqueue
> > 
> > Which virtqueue? It says TX/RX pretty consistently in this text.
> > Changing to receive virtqueue/transmit virtqueue would be a big change and
> > frankly for a very modest gain in readability.
> > Rather maybe just say RX/TX where we describe virtqueue.
> > 
> We are describing rest of the previous sections as transmitq, receiveq.
> Would like to keep this section consistent with rest of the Network device section, and not just notification coalescing section.

Maybe, but let's change one thing at a time.

> > > >
> > > >  The class VIRTIO_NET_CTRL_NOTF_COAL has 2 commands:
> > > >  \begin{enumerate}
> > > > -\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the \field{tx_usecs}
> > > > and \field{tx_max_packets} parameters.
> > > > -\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{rx_usecs}
> > > > and \field{rx_max_packets} parameters.
> > > > +\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the \field{usecs} and
> > > > \field{max_packets} parameters for TX.
> > > > +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{usecs} and
> > > > \field{max_packets} parameters for RX.
> > > >  \end{enumerate}
> > > >
> > > >  \subparagraph{RX Notifications}\label{sec:Device Types / Network
> > > > Device / Device Operation / Control Virtqueue / Notifications
> > > > Coalescing / RX Notifications}
> > > >
> > > >  If, for example:
> > > >  \begin{itemize}
> > > > -\item \field{rx_usecs} = 10.
> > > > -\item \field{rx_max_packets} = 15.
> > > > +\item \field{usecs} = 10.
> > > > +\item \field{max_packets} = 15.
> > > >  \end{itemize}
> > > >
> > > >  The device will operate as follows:
> > > > @@ -1564,8 +1561,8 @@ \subsubsection{Control
> > > > Virtqueue}\label{sec:Device Types / Network Device / Devi
> > > >
> > > >  If, for example:
> > > >  \begin{itemize}
> > > > -\item \field{tx_usecs} = 10.
> > > > -\item \field{tx_max_packets} = 15.
> > > > +\item \field{usecs} = 10.
> > > > +\item \field{max_packets} = 15.
> > > >  \end{itemize}
> > > >
> > > >  The device will operate as follows:
> > > > @@ -1575,6 +1572,13 @@ \subsubsection{Control
> > > > Virtqueue}\label{sec:Device Types / Network Device / Devi  \item If
> > > > the notifications are not suppressed by the driver, the device will
> > > > send an used buffer notification, otherwise, the device will not
> > > > send an used buffer notification as long as the notifications are suppressed.
> > > >  \end{itemize}
> > > >
> > > > +\subparagraph{Notifications When Coalescing Parameters
> > > > +Change}\label{sec:Device Types / Network Device / Device Operation
> > > > +/ Control Virtqueue / Notifications Coalescing / Notifications When
> > > > +Coalescing Parameters Change}
> > > > +
> > > > +When a device changes the coalescing parameters, the device needs
> > > > +to check
> > > > if the new parameters are met and issue a notification if so.
> > > > +
> > > > +For example, \field{max_packets} = 15 for TX.
> > > > +If the device sends 10 packets, then it receives a
> > > > VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command with
> > \field{max_packets} =
> > > > 8, the device needs to immediately send a TX notification, if the
> > > > notifications are not suppressed by the driver.
> > > > +
> > > Its better written as,
> > > When device applies new coalescing parameters, if the new parameters
> > already meet the current packet counters, device SHOULD generate immediate
> > notification.
> > 
> > what are packet counters though? they aren't defined anywhere.
> > 
> Notification coalescing is counting packets in current text as " The device will count sent packets until it accumulates 15 ..."

so it says counts packets but it does not mean there's a counter.
And "meet" does not really work with "parameters"...

> > > For example, current \field{max_packets} is 15 for transmit virtqueue, and
> > device already sent 10 packets.
> > 
> > I assume it is all per virtqueue? Makes sense but it does not actually say
> > anywhere.
> > 
> Yes, it is per virtqueue because the coalescing commands are per virtqueue.

in what sense? there's a single set of parameters per device.
\begin{lstlisting}
struct virtio_net_ctrl_coal_rx {
    le32 rx_max_packets;
    le32 rx_usecs;
};

struct virtio_net_ctrl_coal_tx {
    le32 tx_max_packets;
    le32 tx_usecs;
};

#define VIRTIO_NET_CTRL_NOTF_COAL 6
 #define VIRTIO_NET_CTRL_NOTF_COAL_TX_SET  0
 #define VIRTIO_NET_CTRL_NOTF_COAL_RX_SET 1
\end{lstlisting}

does not specify to which vq this applies, I think the implication
is it applies to all of them.



> Not sure explicitly keep adding per virtqueue in every line improves the readability.

well we have to say this somewhere at least and AFAIK we don't.

> > > When device receives a VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command,
> > the device SHOULD immediately send a TX notification.
> > 
> > always? why?
> Ah no. not always. It was in the context of the example continuation.
> 
> May be better to rewrite as,
> 
> For example, current \field{max_packets} is 15 for the transmit virtqueue, and device already sent 10 packets; in such case when device receives a VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command, the device SHOULD immediately send a TX notification.


Parav this is not the best way to give comments I feel. Either just send
a competing patch or (preferably) explain what is wrong with this one.
Piecemeal "Its better written as" without anything in the way of
explanation is not helpful. I for one don't have the time to review and
reply to not just patches but also all the random suggestions you are
throwing around.

-- 
MST