linux-kernel - Re: Writable module parameters in KVM

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Date:   Wed, 26 May 2021 14:10:10 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Ben Gardon <bgardon@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Subject: Re: Writable module parameters in KVM

On Wed, 2021-05-26 at 12:49 +0200, Paolo Bonzini wrote:
> On 26/05/21 01:45, Ben Gardon wrote:
> > At Google we have an informal practice of adding sysctls to control some 
> > KVM features. Usually these just act as simple "chicken bits" which 
> > allow us to turn off a feature without having to stall a kernel rollout 
> > if some feature causes problems. (Sysctls were used for reasons specific 
> > to Google infrastructure, not because they're necessarily better.)
> > 
> > We'd like to get rid of this divergence with upstream by converting the 
> > sysctls to writable module parameters, but I'm not sure what the general 
> > guidance is on writable module parameters. Looking through KVM, it seems 
> > like we have several writable parameters, but they're mostly read-only.
> 
> Sure, making them writable is okay.  Most KVM parameters are read-only 
> because it's much simpler (the usecase for introducing them was simply 
> "test what would happen on old processors").  What are these features 
> that you'd like to control?
> 
> > I also don't see central documentation of the module parameters. They're 
> > mentioned in the documentation for other features, but don't have their 
> > own section / file. Should they?
> 
> They probably should, yes.
> 
> Paolo
> 
I vote (because I have fun with my win98 once in a while),
to make 'npt' writable, since that is the only way
to make it run on KVM on AMD.
My personal itch only though!

Best regards,
	Maxim Levitsky