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Message-ID: <87o7jq64s4.fsf@intel.com>
Date: Tue, 01 Aug 2023 10:39:23 -0700
From: Vinicius Costa Gomes <vinicius.gomes@...el.com>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, Eric
Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
Abeni <pabeni@...hat.com>, Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang
<xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
linux-kernel@...r.kernel.org, intel-wired-lan@...ts.osuosl.org, Muhammad
Husaini Zulkifli <muhammad.husaini.zulkifli@...el.com>, Peilin Ye
<yepeilin.cs@...il.com>, Pedro Tammela <pctammela@...atatu.com>, Richard
Cochran <richardcochran@...il.com>, Zhengchao Shao
<shaozhengchao@...wei.com>, Maxim Georgiev <glipus@...il.com>
Subject: Re: [PATCH v2 net-next 7/9] net: netdevsim: mimic tc-taprio offload
Hi Vladimir,
Vladimir Oltean <vladimir.oltean@....com> 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/20230130173145.475943-15-vladimir.oltean@nxp.com/
> 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
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