linux-kernel - Re: [PATCH] KVM: introduce vm's max_halt_poll

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Date: Fri, 10 May 2024 07:07:12 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: cheng.lin130@....com.cn
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	jiang.yong5@....com.cn, wang.liang82@....com.cn, jiang.xuexin@....com.cn
Subject: Re: [PATCH] KVM: introduce vm's max_halt_poll_ns to debugfs

On Fri, May 10, 2024, cheng.lin130@....com.cn wrote:
> > > > From: seanjc <seanjc@...gle.com>
> > > > > From: Cheng Lin <cheng.lin130@....com.cn>
> > > > >
> > > > > Introduce vm's max_halt_poll_ns and override_halt_poll_ns to
> > > > > debugfs. Provide a way to check and modify them.
> > > > Why?
> > > If a vm's max_halt_poll_ns has been set using KVM_CAP_HALT_POLL,
> > > the module parameter kvm.halt_poll.ns will no longer indicate the maximum
> > > halt pooling interval for that vm. After introducing these two attributes into
> > > debugfs, it can be used to check whether the individual configuration of the
> > > vm is enabled and the working value.
> > But why is max_halt_poll_ns special enough to warrant debugfs entries?  There is
> > a _lot_ of state in KVM that is configurable per-VM, it simply isn't feasible to
> > dump everything into debugfs.
> If we want to provide a directly modification interface under /sys for per-vm
> max_halt_poll_ns, like module parameter /sys/module/kvm/parameters/halt_poll_ns,
> using debugfs may be worth.

Yes, but _why_?  I know _what_ a debugs knob allows, but you have yet to explain
why this

General speaking, functionality of any kind should not be routed through debugfs,
it really is meant for debug.  E.g. it's typically root-only, is not guaranteed
to exist, its population is best-effort, etc.

> Further, if the override_halt_poll_ns under debugfs is set to be writable, it can even
> achieve the setting of per-vm max_halt_poll_ns, as the KVM_CAP_HALL_POLL interface
> does.
> > I do think it would be reasonable to capture the max allowed polling time in
> > the existing tracepoint though, e.g.
> Yes, I agree it. 
> It is sufficient to get per-vm max_halt_poll_ns through tracepoint if KVP_CAP_HALL_POLL
> is used as the unique setting interface.
> 
> Do you consider it is worth to provide a setting interface other than KVP_CAP_HALL_POLL?