Re: [PATCH v2 2/3] KVM: x86: Harden against unexpected adjustments to kvm_cpu_caps

[Xiaoyao Li]

On 1/29/2026 11:12 PM, Sean Christopherson wrote:
> 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@xxxxxxxxxx>
> > > ---
> > >    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.

And after a second thought, I find my preference is not good. Because it 
only tells the end of allowed stage while kvm_is_configuring_cpu_caps 
defines both the start and end.

So withdraw my preference.