linux-kernel - Re: [PATCH v7 008/102] KVM: x86: Refactor KVM VMX module init/exit functions

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Date:   Tue, 12 Jul 2022 13:30:34 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v7 008/102] KVM: x86: Refactor KVM VMX module init/exit
 functions

On Mon, 2022-07-11 at 17:38 -0700, Isaku Yamahata wrote:
> On Tue, Jun 28, 2022 at 03:53:31PM +1200,
> Kai Huang <kai.huang@...el.com> wrote:
> 
> > On Mon, 2022-06-27 at 14:53 -0700, isaku.yamahata@...el.com wrote:
> > > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > > 
> > > Currently, KVM VMX module initialization/exit functions are a single
> > > function each.  Refactor KVM VMX module initialization functions into KVM
> > > common part and VMX part so that TDX specific part can be added cleanly.
> > > Opportunistically refactor module exit function as well.
> > > 
> > > The current module initialization flow is, 1.) calculate the sizes of VMX
> > > kvm structure and VMX vcpu structure, 2.) hyper-v specific initialization
> > > 3.) report those sizes to the KVM common layer and KVM common
> > > initialization, and 4.) VMX specific system-wide initialization.
> > > 
> > > Refactor the KVM VMX module initialization function into functions with a
> > > wrapper function to separate VMX logic in vmx.c from a file, main.c, common
> > > among VMX and TDX.  We have a wrapper function, "vt_init() {vmx kvm/vcpu
> > > size calculation; hv_vp_assist_page_init(); kvm_init(); vmx_init(); }" in
> > > main.c, and hv_vp_assist_page_init() and vmx_init() in vmx.c.
> > > hv_vp_assist_page_init() initializes hyper-v specific assist pages,
> > > kvm_init() does system-wide initialization of the KVM common layer, and
> > > vmx_init() does system-wide VMX initialization.
> > > 
> > > The KVM architecture common layer allocates struct kvm with reported size
> > > for architecture-specific code.  The KVM VMX module defines its structure
> > > as struct vmx_kvm { struct kvm; VMX specific members;} and uses it as
> > > struct vmx kvm.  Similar for vcpu structure. TDX KVM patches will define
> > > TDX specific kvm and vcpu structures, add tdx_pre_kvm_init() to report the
> > > sizes of them to the KVM common layer.
> > > 
> > > The current module exit function is also a single function, a combination
> > > of VMX specific logic and common KVM logic.  Refactor it into VMX specific
> > > logic and KVM common logic.  This is just refactoring to keep the VMX
> > > specific logic in vmx.c from main.c.
> > 
> > This patch, coupled with the patch:
> > 
> > 	KVM: VMX: Move out vmx_x86_ops to 'main.c' to wrap VMX and TDX
> > 
> > Basically provides an infrastructure to support both VMX and TDX.  Why we cannot
> > merge them into one patch?  What's the benefit of splitting them?
> > 
> > At least, why the two patches cannot be put together closely?
> 
> It is trivial for the change of "KVM: VMX: Move out vmx_x86_ops to 'main.c' to
> wrap VMX and TDX" to introduce no functional change.  But it's not trivial
> for this patch to introduce no functional change.

This doesn't sound right.  If I understand correctly, this patch supposedly
shouldn't bring any functional change, right?  Could you explain what functional
change does this patch bring?