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





Leon Romanovsky 於 2021/3/21 18:22 寫道:
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On Sun, Mar 21, 2021 at 05:31:08PM +0100, Hsu, Chiahao wrote:

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?
At the end, it will be more granular module parameter that user still
will need to guess.
I believe users always need to know any parameter or any tool's flag before they use it. For example, before user try to set/clear this ctrl_ring_enabled, they should already have basic knowledge about this feature, or else they shouldn't use it (the default value is same as before), and that's also why we use the 'ctrl_ring_enabled' as parameter name.

Thanks
Thanks





 


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