linux-kernel - Re: [PATCH v16 29/51] KVM: VMX: Configure nested capabilities after CPU capabilities

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Message-ID: <aNIH/ozYmopOuCui@intel.com>
Date: Tue, 23 Sep 2025 10:37:50 +0800
From: Chao Gao <chao.gao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Tom Lendacky <thomas.lendacky@....com>,
	Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Binbin Wu
	<binbin.wu@...ux.intel.com>, Xiaoyao Li <xiaoyao.li@...el.com>, "Maxim
 Levitsky" <mlevitsk@...hat.com>, Zhang Yi Z <yi.z.zhang@...ux.intel.com>,
	"Xin Li" <xin@...or.com>
Subject: Re: [PATCH v16 29/51] KVM: VMX: Configure nested capabilities after
 CPU capabilities

On Fri, Sep 19, 2025 at 03:32:36PM -0700, Sean Christopherson wrote:
>Swap the order between configuring nested VMX capabilities and base CPU
>capabilities, so that nested VMX support can be conditioned on core KVM
>support, e.g. to allow conditioning support for LOAD_CET_STATE on the
>presence of IBT or SHSTK.  Because the sanity checks on nested VMX config
>performed by vmx_check_processor_compat() run _after_ vmx_hardware_setup(),
>any use of kvm_cpu_cap_has() when configuring nested VMX support will lead
>to failures in vmx_check_processor_compat().
>
>While swapping the order of two (or more) configuration flows can lead to
>a game of whack-a-mole, in this case nested support inarguably should be
>done after base support.  KVM should never condition base support on nested
>support, because nested support is fully optional, while obviously it's
>desirable to condition nested support on base support.  And there's zero
>evidence the current ordering was intentional, e.g. commit 66a6950f9995
>("KVM: x86: Introduce kvm_cpu_caps to replace runtime CPUID masking")
>likely placed the call to kvm_set_cpu_caps() after nested setup because it
>looked pretty.
>
>Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Chao Gao <chao.gao@...el.com>

I had a feeling I'd seen this patch before :). After some searching in lore, I
tracked it down:
https://lore.kernel.org/kvm/20241001050110.3643764-22-xin@zytor.com/