netdev - RE: [net-next 0/4] devlink: add dry run support for flash update

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Message-ID: <CO1PR11MB5089CD14E6293B8024202726D6B59@CO1PR11MB5089.namprd11.prod.outlook.com>
Date:   Mon, 11 Oct 2021 08:21:33 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>, Jiri Pirko <jiri@...nulli.us>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [net-next 0/4] devlink: add dry run support for flash update

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Friday, October 08, 2021 11:22 AM
> To: Jiri Pirko <jiri@...nulli.us>
> Cc: Keller, Jacob E <jacob.e.keller@...el.com>; netdev@...r.kernel.org
> Subject: Re: [net-next 0/4] devlink: add dry run support for flash update
> 
> On Fri, 8 Oct 2021 14:37:37 +0200 Jiri Pirko wrote:
> > Fri, Oct 08, 2021 at 12:41:11PM CEST, jacob.e.keller@...el.com wrote:
> > >This is an implementation of a previous idea I had discussed on the list at
> >
> >https://lore.kernel.org/netdev/51a6e7a33c7d40889c80bf37159f210e@intel.co
> m/
> > >
> > >The idea is to allow user space to query whether a given destructive devlink
> > >command would work without actually performing any actions. This is
> commonly
> > >referred to as a "dry run", and is intended to give tools and system
> > >administrators the ability to test things like flash image validity, or
> > >whether a given option is valid without having to risk performing the update
> > >when not sufficiently ready.
> > >
> > >The intention is that all "destructive" commands can be updated to support
> > >the new DEVLINK_ATTR_DRY_RUN, although this series only implements it for
> > >flash update.
> > >
> > >I expect we would want to support this for commands such as reload as well
> > >as other commands which perform some action with no interface to check
> state
> > >before hand.
> > >
> > >I tried to implement the DRY_RUN checks along with useful extended ACK
> > >messages so that even if a driver does not support DRY_RUN, some useful
> > >information can be retrieved. (i.e. the stack indicates that flash update is
> > >supported and will validate the other attributes first before rejecting the
> > >command due to inability to fully validate the run within the driver).
> >
> > Hmm, old kernel vs. new-userspace, the requested dry-run, won't be
> > really dry run. I guess that user might be surprised in that case...
> 
> Would it be enough to do a policy dump in user space to check attr is
> recognized and add a warning that this is required next to the attr
> in the uAPI header?

I'd be more in favor of converting either this specific op (or devlink as a whole) to strict checking. I think that most of the devlink commands and ops would function better if unknown behaviors were rejected rather than ignored.

If we prefer to avoid that due to historically not being strict, I'm ok with a userspace check. It does complicate the userspace a bit more, but I agree that especially for dry_run we don't want it accidentally updating when a dry run was expected.

There is some maintenance cost to switching to strict checking but I think it's pretty minimal because the strict checking simply prevents the unknown attributes from being ignored.

I'm happy to go either direction if we get consensus on this thread though.

Thanks,
Jake