Re: [PATCH net-next v6 04/15] ethtool: introduce ethtool netlink interface

[Jiri Pirko]

Mon, Jul 08, 2019 at 10:22:19PM CEST, mkubecek@xxxxxxx wrote:
>On Mon, Jul 08, 2019 at 09:26:29PM +0200, Jiri Pirko wrote:
>> Mon, Jul 08, 2019 at 07:27:29PM CEST, mkubecek@xxxxxxx wrote:
>> >
>> >There are two reasons for this design. First is to reduce the number of
>> >requests needed to get the information. This is not so much a problem of
>> >ethtool itself; the only existing commands that would result in multiple
>> >request messages would be "ethtool <dev>" and "ethtool -s <dev>". Maybe
>> >also "ethtool -x/-X <dev>" but even if the indirection table and hash
>> >key have different bits assigned now, they don't have to be split even
>> >if we split other commands. It may be bigger problem for daemons wanting
>> >to keep track of system configuration which would have to issue many
>> >requests whenever a new device appears.
>> >
>> >Second reason is that with 8-bit genetlink command/message id, the space
>> >is not as infinite as it might seem. I counted quickly, right now the
>> >full series uses 14 ids for kernel messages, with split you propose it
>> >would most likely grow to 44. For full implementation of all ethtool
>> >functionality, we could get to ~60 ids. It's still only 1/4 of the
>> >available space but it's not clear what the future development will look
>> >like. We would certainly need to be careful not to start allocating new
>> >commands for single parameters and try to be foreseeing about what can
>> >be grouped together. But we will need to do that in any case.
>> >
>> >On kernel side, splitting existing messages would make some things a bit
>> >easier. It would also reduce the number of scenarios where only part of
>> >requested information is available or only part of a SET request fails.
>> 
>> Okay, I got your point. So why don't we look at if from the other angle.
>> Why don't we have only single get/set command that would be in general
>> used to get/set ALL info from/to the kernel. Where we can have these
>> bits (perhaps rather varlen bitfield) to for user to indicate which data
>> is he interested in? This scales. The other commands would be
>> just for action.
>> 
>> Something like RTM_GETLINK/RTM_SETLINK. Makes sense?
>
>It's certainly an option but at the first glance it seems as just moving
>what I tried to avoid one level lower. It would work around the u8 issue
>(but as Johannes pointed out, we can handle it with genetlink when/if
>the time comes). We would almost certainly have to split the replies
>into multiple messages to keep the packet size reasonable. I'll have to
>think more about the consequences for both kernel and userspace.
>
>My gut feeling is that out of the two extreme options (one universal
>message type and message types corresponding to current infomask bits),
>the latter is more appealing. After all, ethtool has been gathering
>features that would need those ~60 message types for 20 years.

Yeah, but I think that we have to do one or another. Anything in between
makes the code complex and uapi confusing. Let's start clean :)