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Subject: Re: [PATCH v4] virtio-net: support the virtqueue coalescing moderation
On Mon, Feb 20, 2023 at 10:25:49PM +0800, Heng Qi wrote:
>
> å 2023/2/20 äå9:14, Michael S. Tsirkin åé:
> > On Mon, Feb 20, 2023 at 03:44:13PM +0800, Heng Qi wrote:
> > > On Sun, Feb 19, 2023 at 10:06:51AM -0500, Michael S. Tsirkin wrote:
> > > > On Sun, Feb 19, 2023 at 10:45:07PM +0800, Heng Qi wrote:
> > > > > Currently coalescing parameters are grouped for all transmit and receive
> > > > > virtqueues. This patch supports setting or getting the parameters for a
> > > > > specified virtqueue, and a typical application of this function is netdim[1].
> > > > >
> > > > > When the traffic between virtqueues is unbalanced, for example, one virtqueue
> > > > > is busy and another virtqueue is idle, then it will be very useful to
> > > > > control coalescing parameters at the virtqueue granularity.
> > > > >
> > > > > [1] https://docs.kernel.org/networking/net_dim.html
> > > > >
> > > > > Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> > > > > Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > > > ---
> > > > > This patch is on top of Alvaro's latest v7 patch: https://lists.oasis-open.org/archives/virtio-dev/202302/msg00431.html .
> > > > >
> > > > > v3->v4:
> > > > > 1. Include virtio_net_ctrl_coal in the virtio_net_ctrl_coal_vq structure. @Alvaro Karsz
> > > > > 2. Add consideration of vq reset. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
> > > > > 3. Avoid too many examples by giving a comprehensive example. @Michael S. Tsirkin
> > > > > 4. Fix typos and streamline clarifications. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
> > > > >
> > > > > v2->v3:
> > > > > 1. Add the netdim link. @Parav Pandit
> > > > > 2. VIRTIO_NET_F_VQ_NOTF_COAL no longer depends on VIRTIO_NET_F_NOTF_COAL. @Michael S. Tsirkin, @Alvaro Karsz
> > > > > 3. _VQ_GET is explained more. @Michael S. Tsirkin
> > > > > 4. Add more examples to avoid misunderstandings. @Michael S. Tsirkin
> > > > > 5. Clarify some statements. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
> > > > > 6. Adjust the virtio_net_ctrl_coal_vq structure. @Michael S. Tsirkin
> > > > > 7. Fix some typos. @Michael S. Tsirkin
> > > > >
> > > > > v1->v2:
> > > > > 1. Rename VIRTIO_NET_F_PERQUEUE_NOTF_COAL to VIRTIO_NET_F_VQ_NOTF_COAL. @Michael S. Tsirkin
> > > > > 2. Use the \field{vqn} instead of the qid. @Michael S. Tsirkin
> > > > > 3. Unify tx and rx control structres into one structure virtio_net_ctrl_coal_vq. @Michael S. Tsirkin
> > > > > 4. Add a new control command VIRTIO_NET_CTRL_NOTF_COAL_VQ. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
> > > > > 5. The special value 0xFFF is removed because VIRTIO_NET_CTRL_NOTF_COAL can be used. @Alvaro Karsz
> > > > > 6. Clarify some special scenarios. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
> > > > >
> > > > > device-types/net/description.tex | 64 ++++++++++++++++++++++++++++++--
> > > > > 1 file changed, 60 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> > > > > index e71e33b..fa8bd26 100644
> > > > > --- a/device-types/net/description.tex
> > > > > +++ b/device-types/net/description.tex
> > > > > @@ -83,6 +83,8 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
> > > > > \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
> > > > > channel.
> > > > > +\item[VIRTIO_NET_F_VQ_NOTF_COAL(52)] Device supports virtqueue notification coalescing.
> > > > > +
> > > > > \item[VIRTIO_NET_F_NOTF_COAL(53)] Device supports notifications coalescing.
> > > > > \item[VIRTIO_NET_F_GUEST_USO4 (54)] Driver can receive USOv4 packets.
> > > > > @@ -139,6 +141,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
> > > > > \item[VIRTIO_NET_F_NOTF_COAL] Requires VIRTIO_NET_F_CTRL_VQ.
> > > > > \item[VIRTIO_NET_F_RSC_EXT] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
> > > > > \item[VIRTIO_NET_F_RSS] Requires VIRTIO_NET_F_CTRL_VQ.
> > > > > +\item[VIRTIO_NET_F_VQ_NOTF_COAL] Requires VIRTIO_NET_F_CTRL_VQ.
> > > > > \end{description}
> > > > > \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network Device / Feature bits / Legacy Interface: Feature bits}
> > > > > @@ -1508,6 +1511,12 @@ \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.
> > > > > +If the VIRTIO_NET_F_VQ_NOTF_COAL feature is negotiated:
> > > > > +1. a driver can send a VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command to set the coalescing
> > > > > + parameters of an enabled transmit/receive virtqueue.
> > > > > +2. a driver can send a VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET command to a device, and the device
> > > > > + returns the coalescing parameters of an enabled transmit/receive virtqueue.
> > > > > +
> > > > > \begin{note}
> > > > > The behavior of the device in response to these commands is best-effort:
> > > > > the device may generate notifications more or less frequently than specified.
> > > > > @@ -1519,25 +1528,55 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
> > > > > le32 max_usecs;
> > > > > };
> > > > > +struct virtio_net_ctrl_coal_vq {
> > > > > + le16 vqn;
> > > > > + le16 reserved;
> > > > > + struct virtio_net_ctrl_coal coal;
> > > > > +};
> > > > > +
> > > > 2 structures now, what are they doing? which command uses which?
> > > > reader is supposed to guess and that's not good.
> > > >
> > > > Also, with VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET some fields are
> > > > output others are input. This is unusual for commands
> > > > and needs to be documented very clearly.
> > > Yes, this should be made clear. How do you think of the following descriptions:
> > >
> > > "
> > > The VIRTIO_NET_CTRL_NOTF_COAL_{TX, RX}_SET commands use the
> > I think it's better to just say VIRTIO_NET_CTRL_NOTF_COAL_RX_SET and
> > VIRTIO_NET_CTRL_NOTF_COAL_TX_SET
> > We don't document the {} syntax anywhere.
>
> Ok, I'll modify this.
>
> >
> > > virtio_net_ctrl_coal structure to set \field{max_usecs} and
> > > \field{max_packets} for all transmit/receive virtqueues.
> > >
> > > The VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command uses the
> > > virtio_net_ctrl_coal_vq structure to set \field{max_usecs}
> > > and \field{max_packets} for the supplied virtqueue number \field{vqn}.
> > >
> > > The VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET command gets the values of
> > > \field{max_usecs} and \field{max_packets} of the specified
> > > virtqueue from the device by only setting \field{vqn} in virtio_net_ctrl_coal_vq.
> > > "
> >
> > Not enough imho. Document which fields are write-only for driver and
> > which are read-only for driver. and includes reserved field not just
> > vqn.
>
> OK I rephrase it into following sentences (Combined with the following,
> \field{vqn}'s and\field{reserved}'s explanations may be a bit redundant, but
> I will merge them in the next version ) :
>
> "
> \field{max_usecs} and \field{max_packets} are collectively known as
> coalescing parameters.
> \field{vqn} and \field{reserved} are only used by
> VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET
> commands.
ugh just confusing. just use the struct names.
> \field{vqn} points to an enabled transmit/receive virtqueue, and its value
> satisfies $ 0 \leq vqn < max_virtqueue_pairs \ast 2 $.
> Â\field{reserved} is a reserved field containing 0, which is ignored by a
> device and write-only for a driver.
how important is it that devices ignore it?
if yes it needs a normative statement.
>
> VIRTIO_NET_CTRL_NOTF_COAL_RX_SET and VIRTIO_NET_CTRL_NOTF_COAL_TX_SET
> commands use the virtio_net_ctrl_coal structure to set \field{max_usecs} and
> \field{max_packets} for all transmit/receive virtqueues, while coalescing
> parameters are write-only for a driver.
>
> The VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command uses the
> virtio_net_ctrl_coal_vq structure to set \field{max_usecs} and
> \field{max_packets} for the supplied virtqueue number \field{vqn}, while
> coalescing parameters and \field{vqn} are write-only for a driver.
>
> The VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET command gets the values of
> \field{max_usecs} and \field{max_packets} of the specified virtqueue from a
> device by only setting \field{vqn} in virtio_net_ctrl_coal_vq. At this time,
at what time?
> for a driver, coalescing parameters are read-only and \field{vqn} is
> write-only.
reserved also write-only.
> "
> Do you think this is a good description?
No. I think you should take your previous one and explain:
One way:
commands X and Y use structure Z in command specific
data.
for command X fields A B are write-only for driver and
fields C D are read-only for driver.
for command Y fields A B C and D are write-only for driver.
> > > > > #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
> > > > > + #define VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET 2
> > > > > + #define VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET 3
> > > > > \end{lstlisting}
> > > > > Coalescing parameters:
> > > > > \begin{itemize}
> > > > > +\item \field{vqn}: The virtqueue number of an enabled transmit or receive virtqueue.
> > > > > \item \field{max_usecs} for RX: Maximum number of microseconds to delay a RX notification.
> > > > > \item \field{max_usecs} for TX: Maximum number of microseconds 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}
> > > > > -The class VIRTIO_NET_CTRL_NOTF_COAL has 2 commands:
> > > > > +\field{vqn} points to an enabled transmit/receive virtqueue, and its value satisfies $ 0 \leq vqn < max_virtqueue_pairs \ast 2 $.
> > > > > +
> > > > > +\field{reserved} is reserved and it is ignored by the device.
> > > > > +
> > > > > +The class VIRTIO_NET_CTRL_NOTF_COAL has 4 commands:
> > > > > \begin{enumerate}
> > > > > -\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the \field{max_usecs} and \field{max_packets} parameters for all transmit virtqueues.
> > > > > -\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{max_usecs} and \field{max_packets} parameters for all receive virtqueues.
> > > > > +\item VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET: set the \field{max_usecs} and \field{max_packets} parameters for an enabled transmit/receive
> > > > > + virtqueue whose number is \field{vqn}.
> > > > > +\item VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET: the device returns the \field{max_usecs} and \field{max_packets} parameters for an enabled
> > > > > + transmit/receive virtqueue whose number is \field{vqn}.
> > > > > +\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: have the same effect as the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command repeated for each virtqueue of transmitq1\ldots transmitqN.
> > > > > +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: have the same effect as the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command repeated for each virtqueue of receiveq1\ldots receiveqN.
> > > > > \end{enumerate}
> > > > > +If coalescing parameters are being set, the device applies the last coalescing parameters received for a
> > > > > +virtqueue, regardless of the command used to set the parameters. For example with 2 pairs of virtqueues:
> > > > > +# Command sequece
> > > > > +Each of the following commands sets \field{max_usecs} and \field{max_packets} parameters for virtqueues.
> > > > > +\begin{itemize}
> > > > > +\item Command1: VIRTIO_NET_CTRL_NOTF_COAL_RX_SET sets parameters for virtqueue0 and virtqueue2, and, virtqueue1 and virtqueue3 retain their previous parameter values.
> > > > > +\item Command2: VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET with \field{vqn} = 0 sets parameters for virtqueue0, and virtqueue2 retains the values from command1.
> > > > > +\item Command3: VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET with \field{vqn}= 0, the device returns paramters of virtqueue0 set by command2.
> > > > > +\item Command4: VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET with \field{vqn} = 1 sets parameters for virtqueue1, and virtqueue3 retains its previous values.
> > > > > +\item Command5: VIRTIO_NET_CTRL_NOTF_COAL_TX_SET sets parameters for virtqueue1 and virtqueue3, and overrides the values set by command4.
> > > > > +\item Command6: VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET with \field{vqn}= 1, the device returns paramters of virtqueue1 set by command5.
> > > > > +\end{itemize}
> > > > > +
> > > > > \subparagraph{Operation}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / Operation}
> > > > > The device sends a used buffer notification once the notification conditions are met and if the notifications are not suppressed as explained in \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Used Buffer Notification Suppression}.
> > > > > @@ -1585,14 +1624,31 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
> > > > > \drivernormative{\subparagraph}{Notifications Coalescing}{Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing}
> > > > > -If the VIRTIO_NET_F_NOTF_COAL feature has not been negotiated, the driver MUST NOT issue VIRTIO_NET_CTRL_NOTF_COAL commands.
> > > > > +If neither the VIRTIO_NET_F_NOTF_COAL nor the VIRTIO_NET_F_VQ_NOTF_COAL feature
> > > > > +has not been negotiated, the driver MUST NOT issue VIRTIO_NET_CTRL_NOTF_COAL commands.
> > > > Triple negative? I think you mean "has been negotiated".
> > > Yes. Thanks for pointing it out.
> > >
> > > > Generally we should consider rewriting these in a positive way
> > > > but that's a subject for another patch.
> > > Ok, I see.
> > >
> > > > > +
> > > > > +A driver MUST ignore the values of coalescing paramters received from VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET,
> > > > > +if the device returns with VIRTIO_NET_ERR.
> > > > device does not return. It responds to the command.
> > > Ok.
> > >
> > > > > \devicenormative{\subparagraph}{Notifications Coalescing}{Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing}
> > > > > A device SHOULD respond to the VIRTIO_NET_CTRL_NOTF_COAL commands with VIRTIO_NET_ERR if it was not able to change the parameters.
> > > > > +Note: VIRTIO_NET_CTRL_NOTF_COAL_VQ_{SET, GET} are exception to it, which must return VIRTIO_NET_ERR if the device was unable to change the parameters.
> > > > I don't get it. what is exception to what and what's the difference,
> > > > what does "which" refer to what return VIRTIO_NET_ERR where.
> > > >
> > > Sorry for not describing clearly, please see the explanation below:
> > > When a device receives VIRTIO_NET_CTRL_NOTF_COAL_VQ_{SET, GET} and VIRTIO_NET_CTRL_NOTF_COAL_{TX, RX}_SET respectively,
> > > and the device was not able to change the parameters, then it behaves differently for VIRTIO_NET_CTRL_NOTF_COAL_VQ_{SET, GET} and VIRTIO_NET_CTRL_NOTF_COAL_{TX, RX}_SET, ie SHOULD and MUST.
> > > For example:
> > > 1. VIRTIO_NET_CTRL_NOTF_COAL_RX_SET sets parameters for all transmit/receive virtqueues, but if only receiveq1 fails to be set, and receiveq2\ldots receiveqN are set successfully,
> > > then the device *SHOULD* respond VIRTIO_NET_ERR for VIRTIO_NET_CTRL_NOTF_COAL_RX_SET.
> > > 2. VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET sets parameters for the virtqueue whose number is \field{vqn}, if the device fails to set, the device *MUST* respond VIRTIO_NET_ERR for VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET.
> > So document the two types separately. Change VIRTIO_NET_CTRL_NOTF_COAL
> > to VIRTIO_NET_CTRL_NOTF_COAL_RX_SET and VIRTIO_NET_CTRL_NOTF_COAL_TX_SET,
> > and add a new sentence for VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET.
>
> Ok.
>
> >
> > And VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET does not change anything so I have
> > no idea why mention it at all.
>
> Ah, you mean the scope of MUST is included in SHOULD?
No I mean just list commands explicitly. Do not make reader guess
what does VIRTIO_NET_CTRL_NOTF_COAL mean when there is no
command like this.
> If you think it's better to remove the *Note* above, I'm fine.
>
> >
> >
> >
> > > > > A device SHOULD NOT send used buffer notifications to the driver if the notifications are suppressed, even if the notification conditions are met.
> > > > > +If a virtqueue is disabled, the device MUST clear its parameters configuration, and:
> > > > parameters here meanong coalescing parameters? It won't be clear to
> > > Yes, I'll write clearly.
> > >
> > > > readers who see the full spec not the patch.
> > > >
> > > > > +1. if the device has received a global parameter values sent by VIRTIO_NET_CTRL_NOTF_{RX, TX}_SET:
> > > > "a" does not go with the plural "values".
> > > Ok.
> > >
> > > > > +after the virtqueue is enabled, its parameters SHOULD be set to the global values.
> > > >
> > > > this is the first you mention global.
> > > I mentioned "global" in the first sentence, but that might not seem very clear.
> > >
> > > > > +2. if the device hasn't received the VIRTIO_NET_CTRL_NOTF_{RX, TX}_SET commands:
> > > > > +regardless of whether the device has received a VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command, after the
> > > > > +virtqueue is enabled, its parameters SHOULD be set to 0.
> > > > Sentence too long at this point I can't say what happens after what.
> > > >
> > > >
> > > > I feel this enumeration of cases is confusing. Would be better to find
> > > > the logic behind all this and document.
> > > >
> > > > For example, we could explain that
> > > >
> > > >
> > > > VIRTIO_NET_CTRL_NOTF_{RX, TX}_SET set
> > > > global values for coalescing parameters for RX and TX respectively and
> > > > apply these global values to all VQs of the given type.
> > > > Global values after device reset are 0.
> > > > VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET sets the coalescing parameters for
> > > > a given enabled VQ without changing the global values.
> > > > Fails on a disabled VQ. Disabling and
> > > > re-enabling a VQ reverts its coalescing parameters back to global values.
> > >
> > > Thanks for your example, I tried to regularize it, do you think it works?
> > >
> > > "
> > > The VIRTIO_NET_CTRL_NOTF_{RX, TX}_SET commands set coalescing parameters for all transmit/receive
> > > virtqueues respectively and values of these coalescing parameters are recorded as *global values* by the device.
> > > The device MUST set the global values of coalescing parameters to 0 after being reset.
> > > The VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command sets the coalescing parameters for a given enabled virtqueue without changing the global values.
> > > After disabling and re-enabling a virtqueue, the deivce MUST revert coalescing parameters of the virtqueue back to global values.
> > ok more or less. is this in normative section? If not avoid MUST.
>
> Yes. It's in normative section. Thanks for your comments!
then pls also add a non normative explanatory informal description.
> >
> > And maybe add text in a normative section too. a bit of repetition there
> > is ok.
> >
> >
> > > "
> > >
> > > >
> > > >
> > > > > +
> > > > > +When a device receives a virtqueue command to set or return coalescing parameters for the supplied \field{vqn},
> > > > > +if the virtqueue is not enabled, the device MUST respond to the command with VIRTIO_NET_ERR.
> > > > > +
> > > > > +A device MAY set the coalescing parameter to a value close to a power of 2 value.
> > > > > +
> > > > > Upon reset, a device MUST initialize all coalescing parameters to 0.
> > > > > \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
> > > > > --
> > > > > 2.19.1.6.gb485710b
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