linux-kernel - Re: [PATCH 2/2] KVM: SVM: Raise #UD if VMMCALL instruction is not intercepted

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Message-ID: <aV1bC3Wk-LbP1hUZ@google.com>
Date: Tue, 6 Jan 2026 10:57:15 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: chengkev@...gle.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	pbonzini@...hat.com, yosry.ahmed@...ux.dev
Subject: Re: [PATCH 2/2] KVM: SVM: Raise #UD if VMMCALL instruction is not intercepted

On Tue, Jan 06, 2026, Andrew Cooper wrote:
> > Mentioning L2 and L1 is confusing.  It reads like arbitrary KVM behavior.  And
> > IMO the most notable thing is what's missing: an intercept check.  _That_ is
> > worth commenting, e.g.
> >
> > 	/*
> > 	 * VMMCALL #UDs if it's not intercepted, and KVM reaches this point if
> > 	 * and only if the VMCALL intercept is not set in vmcb12.
> > 	 */
> 
> Not intercepting VMMCALL is stated to be an unconditional VMRUN
> failure.  APM Vol3 15.5 Canonicalization and Consistency Checks.

Hrm, I can't find that.  I see:

  The VMRUN intercept bit is clear.

but I don't see anything about VMMCALL being a mandatory intercept.

> 
> The "VMMCALL was not intercepted" condition is probably what the
> pipeline really checks, but really it means "in root mode".
> 
> In most nested virt scenarios, L1 knows it's in a VM and can use VMMCALL
> for host facilities.
> 
> ~Andrew