linux-kernel - Re: [PATCH v2 2/3] KVM: x86: Harden against unexpected adjustments to kvm_cpu

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Message-ID: <aXt4wf6Vpo5da2rc@google.com>
Date: Thu, 29 Jan 2026 07:12:01 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Chao Gao <chao.gao@...el.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 2/3] KVM: x86: Harden against unexpected adjustments to kvm_cpu_caps

On Thu, Jan 29, 2026, Xiaoyao Li wrote:
> On 1/28/2026 9:43 AM, Sean Christopherson wrote:
> > Add a flag to track when KVM is actively configuring its CPU caps, and
> > WARN if a cap is set or cleared if KVM isn't in its configuration stage.
> > Modifying CPU caps after {svm,vmx}_set_cpu_caps() can be fatal to KVM, as
> > vendor setup code expects the CPU caps to be frozen at that point, e.g.
> > will do additional configuration based on the caps.
> > 
> > Rename kvm_set_cpu_caps() to kvm_initialize_cpu_caps() to pair with the
> > new "finalize", and to make it more obvious that KVM's CPU caps aren't
> > fully configured within the function.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   arch/x86/kvm/cpuid.c   | 10 ++++++++--
> >   arch/x86/kvm/cpuid.h   | 12 +++++++++++-
> >   arch/x86/kvm/svm/svm.c |  4 +++-
> >   arch/x86/kvm/vmx/vmx.c |  4 +++-
> >   4 files changed, 25 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 575244af9c9f..7fe4e58a6ebf 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -36,6 +36,9 @@
> >   u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
> >   EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_cpu_caps);
> > +bool kvm_is_configuring_cpu_caps __read_mostly;
> 
> I prefer the name, kvm_cpu_caps_finalized. But not strongly, so

"finalized" reads too much like the helper queries if the caps are already
finalized, i.e. like an accessor.