| Message ID | 20210408133829.2135103-1-petrm@nvidia.com |
|---|---|
| Headers | show |
| Series | tc: Introduce a trap-and-forward action | expand |
On Thu, 8 Apr 2021 10:05:07 -0400 Jamal Hadi Salim wrote: > On 2021-04-08 9:38 a.m., Petr Machata wrote: > > The TC action "trap" is used to instruct the HW datapath to drop the > > matched packet and transfer it for processing in the SW pipeline. If > > instead it is desirable to forward the packet and transferring a _copy_ to > > the SW pipeline, there is no practical way to achieve that. > > > > To that end add a new generic action, trap_fwd. In the software pipeline, > > it is equivalent to an OK. When offloading, it should forward the packet to > > the host, but unlike trap it should not drop the packet. > > I am concerned about adding new opcodes which only make sense if you > offload (or make sense only if you are running in s/w). > > Those opcodes are intended to be generic abstractions so the dispatcher > can decide what to do next. Adding things that are specific only > to scenarios of hardware offload removes that opaqueness. > I must have missed the discussion on ACT_TRAP because it is the > same issue there i.e shouldnt be an opcode. For details see: > https://people.netfilter.org/pablo/netdev0.1/papers/Linux-Traffic-Control-Classifier-Action-Subsystem-Architecture.pdf > > IMO: > It seems to me there are two actions here encapsulated in one. > The first is to "trap" and the second is to "drop". > This is no different semantically than say "mirror and drop" > offload being enunciated by "skip_sw". > > Does the spectrum not support multiple actions? > e.g with a policy like: > match blah action trap action drop skip_sw To make sure I understand - are you saying that trap should become more general and support both "and then drop" as well as "and then pass" semantics? Seems like that ship has sailed, but also - how does it make it any better WRT not having HW only opcodes? Or are you saying one is better than two?
Jamal Hadi Salim <jhs@mojatatu.com> writes: > I am concerned about adding new opcodes which only make sense if you > offload (or make sense only if you are running in s/w). > > Those opcodes are intended to be generic abstractions so the dispatcher > can decide what to do next. Adding things that are specific only > to scenarios of hardware offload removes that opaqueness. > I must have missed the discussion on ACT_TRAP because it is the > same issue there i.e shouldnt be an opcode. For details see: > https://people.netfilter.org/pablo/netdev0.1/papers/Linux-Traffic-Control-Classifier-Action-Subsystem-Architecture.pdf Trap has been in since 4.13, so 2017ish. It's done and dusted at this point. > IMO: > It seems to me there are two actions here encapsulated in one. > The first is to "trap" and the second is to "drop". > > This is no different semantically than say "mirror and drop" > offload being enunciated by "skip_sw". > > Does the spectrum not support multiple actions? > e.g with a policy like: > match blah action trap action drop skip_sw Trap drops implicitly. We need a "trap, but don't drop". Expressed in terms of existing actions it would be "mirred egress redirect dev $cpu_port". But how to express $cpu_port except again by a HW-specific magic token I don't know.
On 2021-04-08 5:25 p.m., Jakub Kicinski wrote: > On Thu, 8 Apr 2021 10:05:07 -0400 Jamal Hadi Salim wrote: >> On 2021-04-08 9:38 a.m., Petr Machata wrote: >>> The TC action "trap" is used to instruct the HW datapath to drop the >>> matched packet and transfer it for processing in the SW pipeline. If >>> instead it is desirable to forward the packet and transferring a _copy_ to >>> the SW pipeline, there is no practical way to achieve that. >>> >>> To that end add a new generic action, trap_fwd. In the software pipeline, >>> it is equivalent to an OK. When offloading, it should forward the packet to >>> the host, but unlike trap it should not drop the packet. >> >> I am concerned about adding new opcodes which only make sense if you >> offload (or make sense only if you are running in s/w). >> >> Those opcodes are intended to be generic abstractions so the dispatcher >> can decide what to do next. Adding things that are specific only >> to scenarios of hardware offload removes that opaqueness. >> I must have missed the discussion on ACT_TRAP because it is the >> same issue there i.e shouldnt be an opcode. For details see: >> https://people.netfilter.org/pablo/netdev0.1/papers/Linux-Traffic-Control-Classifier-Action-Subsystem-Architecture.pdf >> >> IMO: >> It seems to me there are two actions here encapsulated in one. >> The first is to "trap" and the second is to "drop". >> This is no different semantically than say "mirror and drop" >> offload being enunciated by "skip_sw". >> >> Does the spectrum not support multiple actions? >> e.g with a policy like: >> match blah action trap action drop skip_sw > > To make sure I understand - are you saying that trap should become > more general and support both "and then drop" as well as "and then > pass" semantics? > No. Main issue is the pollution of the opcodes - whether it is one or multiple actions is less of a concern. Those opcodes are intended to be for the core action dispatcher's consumption. See figure 6 and table 1 of the document i referred to. Basically: You dont an action then add an opcode for it even if it is hardware offloaded (otherwise that opcode space would have grown a lot more by now for all those actions that are offloaded). Trap, for example, could have been a dummy action that just returns the STOLEN/DROP/PASS opcode and does nothing else. Typically we expect things that are offloaded to have a software equivalent. It makes for good control consistency etc clean. > Seems like that ship has sailed, but also - how does it make it any > better WRT not having HW only opcodes? Or are you saying one is better > than two? The opcodes are not tied to whether an action is offloaded or not. That role belongs to the "skip_sw" axes - which works well today since we dont offload actions on their own without some filter rule which specifies the offload option. I will barf if someone implements 3 actions: "trap", "trap and forward", "trap and drop" - but that is not messing up with the core architecture so the barfing is more due to the bad taste of that approach. A cleaner approach is to code one and change the return code for those 3 to "STOLEN", "PIPE", and "DROP" cheers, jamal
On 2021-04-09 7:03 a.m., Petr Machata wrote: > > Jamal Hadi Salim <jhs@mojatatu.com> writes: > >> I am concerned about adding new opcodes which only make sense if you >> offload (or make sense only if you are running in s/w). >> >> Those opcodes are intended to be generic abstractions so the dispatcher >> can decide what to do next. Adding things that are specific only >> to scenarios of hardware offload removes that opaqueness. >> I must have missed the discussion on ACT_TRAP because it is the >> same issue there i.e shouldnt be an opcode. For details see: >> https://people.netfilter.org/pablo/netdev0.1/papers/Linux-Traffic-Control-Classifier-Action-Subsystem-Architecture.pdf > > Trap has been in since 4.13, so 2017ish. It's done and dusted at this > point. > I am afraid that is not a good arguement. With all due respect, here's how it translates: "We already made a mistake, therefore, its ok to build on it and make more mistakes". Touching those opcodes is really dirty; at least i have seen no convincing arguement _at all_ for it. And, it is not too late not to make more mistakes. I dont remember, I may have spoken against TRAP; what i know is had i seen the patch i would have said something - maybe i did and should have been louder. Mea culpa. >> IMO: >> It seems to me there are two actions here encapsulated in one. >> The first is to "trap" and the second is to "drop". >> >> This is no different semantically than say "mirror and drop" >> offload being enunciated by "skip_sw". >> >> Does the spectrum not support multiple actions? >> e.g with a policy like: >> match blah action trap action drop skip_sw > > Trap drops implicitly. We need a "trap, but don't drop". Expressed in > terms of existing actions it would be "mirred egress redirect dev > $cpu_port". But how to express $cpu_port except again by a HW-specific > magic token I don't know. Note: mirred was originally intended to send redirect/mirror packets to user space (the comment is still there in the code). Infact there is a patch lying around somewhere that does that with packet sockets (the author hasnt been serious about pushing it upstream). In that case the semantics are redirecting to a file descriptor. Could we have something like that here which points to whatever representation $cpu_port has? Sounds like semantics for "trap and forward" are just "mirror and forward". I think there is value in having something like trap action which generalizes the combinations only to the fact that it will make it easier to relay the info to the offload without much transformation. If i was to do it i would write one action configured by user space: - to return DROP if you want action trap-and-drop semantics. - to return STOLEN if you want trap - to return PIPE if you want trap and forward. You will need a second action composed to forward. I said dummy because this action has no value in s/w. Someone could use it in s/w but it would be no different than gact. Maybe it could be extended to work also in s/w by adding the "trap to fd" in userspace. cheers, jamal
Jamal Hadi Salim <jhs@mojatatu.com> writes: > On 2021-04-09 7:03 a.m., Petr Machata wrote: >> Jamal Hadi Salim <jhs@mojatatu.com> writes: >> >>> I am concerned about adding new opcodes which only make sense if you >>> offload (or make sense only if you are running in s/w). >>> >>> Those opcodes are intended to be generic abstractions so the dispatcher >>> can decide what to do next. >>> [...] >>> For details see: >>> https://people.netfilter.org/pablo/netdev0.1/papers/Linux-Traffic-Control-Classifier-Action-Subsystem-Architecture.pdf >> >> Trap has been in since 4.13, so 2017ish. It's done and dusted at this >> point. > > here's how it translates: > "We already made a mistake, therefore, its ok to build on it and > make more mistakes". I can see how it reads that way, but that was not the intention. I was actually thinking about whether there might be a way to gradually migrate all this stuff over to mirred, but at this point, trap is very much baked in. >>> IMO: >>> It seems to me there are two actions here encapsulated in one. >>> The first is to "trap" and the second is to "drop". >>> >>> This is no different semantically than say "mirror and drop" >>> offload being enunciated by "skip_sw". >>> >>> Does the spectrum not support multiple actions? >>> e.g with a policy like: >>> match blah action trap action drop skip_sw >> Trap drops implicitly. We need a "trap, but don't drop". Expressed in >> terms of existing actions it would be "mirred egress redirect dev >> $cpu_port". But how to express $cpu_port except again by a HW-specific >> magic token I don't know. (I meant mirred egress mirror, not redirect.) > Note: mirred was originally intended to send redirect/mirror > packets to user space (the comment is still there in the code). > Infact there is a patch lying around somewhere that does that with > packet sockets (the author hasnt been serious about pushing it > upstream). In that case the semantics are redirecting to a file > descriptor. Could we have something like that here which points > to whatever representation $cpu_port has? Sounds like semantics > for "trap and forward" are just "mirror and forward". Hmm, we have devlink ports, the CPU port is exposed there. But that's the only thing that comes to mind. Those are specific for the given device though, it doesn't look suitable... > I think there is value in having something like trap action > which generalizes the combinations only to the fact that > it will make it easier to relay the info to the offload without > much transformation. > If i was to do it i would write one action configured by user space: > - to return DROP if you want action trap-and-drop semantics. > - to return STOLEN if you want trap > - to return PIPE if you want trap and forward. You will need a second > action composed to forward. I think your STOLEN and PIPE are the same behavior. Both are "transfer the packet to the SW datapath, but keep it in the HW datapath". In general I have no issue expressing this stuff as a new action, instead of an opcode. I'll take a look at this.
On 2021-04-09 9:43 a.m., Petr Machata wrote: > > Jamal Hadi Salim <jhs@mojatatu.com> writes: > >>>> Does the spectrum not support multiple actions? >>>> e.g with a policy like: >>>> match blah action trap action drop skip_sw >>> Trap drops implicitly. We need a "trap, but don't drop". Expressed in >>> terms of existing actions it would be "mirred egress redirect dev >>> $cpu_port". But how to express $cpu_port except again by a HW-specific >>> magic token I don't know. > > (I meant mirred egress mirror, not redirect.) > Ok. >> Note: mirred was originally intended to send redirect/mirror >> packets to user space (the comment is still there in the code). >> Infact there is a patch lying around somewhere that does that with >> packet sockets (the author hasnt been serious about pushing it >> upstream). In that case the semantics are redirecting to a file >> descriptor. Could we have something like that here which points >> to whatever representation $cpu_port has? Sounds like semantics >> for "trap and forward" are just "mirror and forward". > > Hmm, we have devlink ports, the CPU port is exposed there. But that's > the only thing that comes to mind. Those are specific for the given > device though, it doesn't look suitable... > If it has an ifindex should be good enough for abstraction purposes. >> I think there is value in having something like trap action >> which generalizes the combinations only to the fact that >> it will make it easier to relay the info to the offload without >> much transformation. >> If i was to do it i would write one action configured by user space: >> - to return DROP if you want action trap-and-drop semantics. >> - to return STOLEN if you want trap >> - to return PIPE if you want trap and forward. You will need a second >> action composed to forward. > > I think your STOLEN and PIPE are the same behavior. Both are "transfer > the packet to the SW datapath, but keep it in the HW datapath". > > In general I have no issue expressing this stuff as a new action, > instead of an opcode. I'll take a look at this. > Ok, thanks. cheers, jamal