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: Wed, 29 May 2024 16:07:58 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.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>, Chao Gao <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, 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.