linux-kernel - Re: [PATCH v2 3/6] KVM: Add a module param to allow enabling virtualization when KVM is loaded

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Date: Thu, 30 May 2024 00:06:50 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Gao, Chao" <chao.gao@...el.com>
Subject: Re: [PATCH v2 3/6] KVM: Add a module param to allow enabling
 virtualization when KVM is loaded

On Wed, 2024-05-29 at 16:07 -0700, Sean Christopherson wrote:
> On Wed, May 29, 2024, Kai Huang wrote:
> > On Wed, 2024-05-29 at 08:01 -0700, Sean Christopherson wrote:
> > > Enabling virtualization should be entirely transparent to userspace,
> > > at least from a functional perspective; if changing how KVM enables virtualization
> > > breaks userspace then we likely have bigger problems.
> > 
> > I am not sure how should I interpret this?
> > 
> > "having a module param" doesn't necessarily mean "entirely transparent to
> > userspace", right? :-)
> 
> Ah, sorry, that was unclear.  By "transparent to userspace" I meant the
> functionality of userspace VMMs wouldn't be affected if we add (or delete) a
> module param.  E.g. QEMU should work exactly the same regardless of when KVM
> enables virtualization.
> 
> > > Performance is secondary for me, the primary motivation is simplifying the overall
> > > KVM code base.  Yes, we _could_ use on_each_cpu() and enable virtualization
> > > on-demand for TDX, but as above, it's extra complexity without any meaningful
> > > benefit, at least AFAICT.
> > 
> > Either way works for me.
> > 
> > I just think using a module param to resolve some problem while there can
> > be solution completely in the kernel seems overkill :-)
> 
> The module param doesn't solve the problem, e.g. we could solve this entirely
> in-kernel simply by having KVM unconditionally enable virtualization during
> initialization.  The module param is mostly there to continue playing nice with
> out-of-tree hypervisors, and to a lesser extent to give us a "break in case of
> fire" knob.

OK.  Now I understand you want to make enabling virtualization at loading
time as default behaviour, but make the module param as a way to defer to
enable virt when creating the first VM.