linux-kernel - Re: [RFC PATCH v3 47/59] KVM: TDX: Define TDCALL exit reason

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Date:   Mon, 29 Nov 2021 18:36:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     isaku.yamahata@...el.com, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, isaku.yamahata@...il.com,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [RFC PATCH v3 47/59] KVM: TDX: Define TDCALL exit reason

On Thu, Nov 25, 2021, Thomas Gleixner wrote:
> On Wed, Nov 24 2021 at 16:20, isaku yamahata wrote:
> > From: Sean Christopherson <sean.j.christopherson@...el.com>
> >
> > Define the TDCALL exit reason, which is carved out from the VMX exit
> > reason namespace as the TDCALL exit from TDX guest to TDX-SEAM is really
> > just a VM-Exit.
> 
> How is this carved out? What's the value of this word salad?
> 
> It's simply a new exit reason. Not more, not less. So what?

The changelog is alluding to the fact that KVM should never directly see a TDCALL
VM-Exit.  For TDX, KVM deals only with "returns" from the TDX-Module.  The "carved
out" bit is calling out that the transition from SEAM Non-Root (the TDX guest) to
SEAM Root (the TDX Module) is actually a VT-x/VMX VM-Exit, e.g. if TDX were somehow
implemented without relying on VT-x/VMX, then the TDCALL exit reason wouldn't exist.

> > Co-developed-by: Xiaoyao Li <xiaoyao.li@...el.com>
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> I'm pretty sure that it does not take two engineers to add a new exit
> reason define, but it takes at least two engineers to come up with a
> convoluted explanation for it.

Nah, just one ;-)