Re: [net-next 1/2] xen-netback: add module parameter to disable ctrl-ring

["Hsu, Chiahao" <andyhsu@xxxxxxxxxx>]

Leon Romanovsky 於 2021/3/17 18:22 寫道:
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> On Tue, Mar 16, 2021 at 04:22:21PM +0100, Hsu, Chiahao wrote:
> > Leon Romanovsky 於 2021/3/14 11:04 寫道:
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> > > On Fri, Mar 12, 2021 at 09:36:59PM +0100, Andrew Lunn wrote:
> > > > On Fri, Mar 12, 2021 at 04:18:02PM +0100, Hsu, Chiahao wrote:
> > > > > Andrew Lunn 於 2021/3/12 15:52 寫道:
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> > > > > > On Thu, Mar 11, 2021 at 10:59:44PM +0000, ChiaHao Hsu wrote:
> > > > > > > In order to support live migration of guests between kernels
> > > > > > > that do and do not support 'feature-ctrl-ring', we add a
> > > > > > > module parameter that allows the feature to be disabled
> > > > > > > at run time, instead of using hardcode value.
> > > > > > > The default value is enable.
> > > > > > Hi ChiaHao
> > > > > >
> > > > > > There is a general dislike for module parameters. What other mechanisms
> > > > > > have you looked at? Would an ethtool private flag work?
> > > > > >
> > > > > >         Andrew
> > > > > Hi Andrew,
> > > > >
> > > > > I can survey other mechanisms, however before I start doing that,
> > > > >
> > > > > could you share more details about what the problem is with using module
> > > > > parameters? thanks.
> > > > It is not very user friendly. No two kernel modules use the same
> > > > module parameters. Often you see the same name, but different
> > > > meaning. There is poor documentation, you often need to read the
> > > > kernel sources it figure out what it does, etc.
> > > +1, It is also global parameter to whole system/devices that use this
> > > module, which is rarely what users want.
> > >
> > > Thanks
> > Hi,
> > I think I would say the current implementation(modparams) isappropriate
> > after reviewing it again.
> >
> > We are talking about 'feature leveling', a way to support live migrationof
> > guest
> > between kernels that do and do not support the features. So we want to
> > refrain
> > fromadding the features if guest would be migrated to the kernel which does
> > not support the feature. Everythingshould be done (in probe function) before
> > frontend connects, and this is why ethtool is not appropriate for this.
> It wouldn't be a surprise to you that feature discovery is not supposed
> to be done through module parameters. Instead of asking from user to
> randomly disable some feature, the system is expected to be backward
> compatible and robust enough to query the list of supported/needed
> features.
>
> Thanks
>
> > Thanks
Typically there should be one VM running netback on each host,
and having control over what interfaces or features it exposes is also 
important for stability.
How about we create a 'feature flags' modparam, each bits is specified 
for different new features?