Subject: Re: [PATCH v2 net-next 7/9] net: netdevsim: mimic tc-taprio offload

From: Vinicius Costa Gomes <[email-masked]>
Date: Tue, 01 Aug 2023 10:39:23 -0700

Hi Vladimir,

Vladimir Oltean <[email-masked]> writes:

> On Wed, Jun 14, 2023 at 05:06:24PM -0700, Vinicius Costa Gomes wrote:
>> > +static int nsim_setup_tc_taprio(struct net_device *dev,
>> > +				struct tc_taprio_qopt_offload *offload)
>> > +{
>> > +	int err = 0;
>> > +
>> > +	switch (offload->cmd) {
>> > +	case TAPRIO_CMD_REPLACE:
>> > +	case TAPRIO_CMD_DESTROY:
>> > +		break;
>> 
>> I was thinking about how useful would proper validation of the
>> parameters be? Thinking that we could detect "driver API" breakages
>> earlier, and we want it documented that the drivers should check for the
>> things that it supports.
>> 
>> Makes sense?
>
> Sorry, I lack imagination as to what the netdevsim driver may check for.
> The taprio offload parameters should always be valid, properly speaking,
> otherwise the Qdisc wouldn't be passing them on to the driver. At least
> that would be the intention. The rest are hardware specific checks for
> hardware specific limitations. Here there is no hardware.
>

Trying to remember what was going through my mind when I said that.

What I seem to recall is something that would help us "keep honest":
I was worrying about someone (perhaps myself ;-) sneaking a new feature
in taprio and forgetting to update other drivers.

I thought that adding a check for the existing parameters would help
detect those kind of things. If anything unknown was there in the
offload struct, netdevsim would complain loudly.

Perhaps I was worrying too much. And the way to solve that is to keep
active attention against that during review.

> The parameters passed to TAPRIO_CMD_REPLACE are:
>
> struct tc_mqprio_qopt_offload mqprio:
> 	struct tc_mqprio_qopt qopt: validated by taprio_parse_mqprio_opt() for flags 0x2
> 	u16 mode: always set to TC_MQPRIO_MODE_DCB
> 	u16 shaper: always set to TC_MQPRIO_SHAPER_DCB
> 	u32 flags: always set to 0
> 	u64 min_rate[TC_QOPT_MAX_QUEUE]: always set to [0,]
> 	u64 max_rate[TC_QOPT_MAX_QUEUE]: always set to [0,]
> 	unsigned long preemptible_tcs: always set to 0, because ethtool_dev_mm_supported() returns false
>
> ktime_t base_time: any value is valid
>
> u64 cycle_time: any value is valid
>
> u64 cycle_time_extension: any value <= cycle_time is valid. According to 802.1Q
> 			  "Q.5 CycleTimeExtension variables", it's the maximum
> 			  amount by which the penultimate cycle can be extended
> 			  to avoid a very short cycle upon a ConfigChange event.
> 			  But if CycleTimeExtension is larger than one CycleTime,
> 			  then we're not even talking about the penultimate cycle
> 			  anymore, but about ones previous to that?! Maybe this
> 			  should be limited to 0 <= cycle_time_extension <= cycle_time
> 			  by taprio, certainly not by offloading drivers.
>

Good point. I have to review 802.1Q, but from what I remember that
sounds right, cycle_time_extension greater than cycle_time doesn't make
much sense. Having a check for it in taprio itself sounds good.

> u32 max_sdu[TC_MAX_QUEUE]: limited to a value <= dev->max_mtu by taprio
>
> size_t num_entries: any value is valid
>
> struct tc_taprio_sched_entry entries[]:
> 	u8 command: will be either one of: TC_TAPRIO_CMD_SET_GATES, TC_TAPRIO_CMD_SET_AND_HOLD
> 		    or TC_TAPRIO_CMD_SET_AND_RELEASE. However 802.1Q "Table 8-7—Gate operations"
> 		    says "If frame preemption is not supported or not enabled (preemptionActive is
> 		    FALSE), this operation behaves the same as SetGateStates.". So I
> 		    see no reason to enforce any restriction here either?
>
> 	u32 gate_mask: technically can have bits set, which correspond
> 		       to traffic classes larger than dev->num_tc.
> 		       Taprio can enforce this, so I wouldn't see
> 		       drivers beginning to feel paranoid about it.
> 		       Actually I had a patch about this:
> 		       https://patchwork.kernel.org/project/netdevbpf/patch/[email-masked]/
> 		       but I decided to drop it because I didn't have
> 		       any strong case for it.
> 	u32 interval: any value is valid. If the sum of entry intervals
> 		      is less than the cycle_time, again that's taprio's
> 		      problem to check for, in its netlink attribute
> 		      validation method rather than offloading drivers.
>

Thank you for the time it took to give this amount of detail.

Cheers,
-- 
Vinicius