Re: [PATCH v2 2/4] KVM: x86: Add LVTCMCI support.

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 12, 2022, Jue Wang wrote:
> This feature is only enabled when the vCPU has opted in to enable
> MCG_CMCI_P.

Again, waaaay too terse.  What is CMCI?  What does "support" mean since an astute
reader will notice that it's impossible for MCG_CMCI_P to be set.  How/when will
the vCPU (which is wrong no?  doesn't userspace do the write?) be able to opt-in?

> Signed-off-by: Jue Wang <juew@google.com>
> ---
>  arch/x86/kvm/lapic.c | 33 ++++++++++++++++++++++++++-------
>  arch/x86/kvm/lapic.h |  7 ++++++-
>  2 files changed, 32 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 2c770e4c0e6c..0b370ccd11a1 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -27,6 +27,7 @@
>  #include <linux/math64.h>
>  #include <linux/slab.h>
>  #include <asm/processor.h>
> +#include <asm/mce.h>
>  #include <asm/msr.h>
>  #include <asm/page.h>
>  #include <asm/current.h>
> @@ -364,9 +365,14 @@ static inline int apic_lvt_nmi_mode(u32 lvt_val)
>  	return (lvt_val & (APIC_MODE_MASK | APIC_LVT_MASKED)) == APIC_DM_NMI;
>  }
>  
> +static inline bool kvm_is_cmci_supported(struct kvm_vcpu *vcpu)
> +{
> +	return vcpu->arch.mcg_cap & MCG_CMCI_P;
> +}
> +
>  static inline int kvm_apic_get_nr_lvt_entries(struct kvm_vcpu *vcpu)

I think it makes sense to take @apic here, not @vcpu, since this is an APIC-specific
helper.  kvm_apic_set_version() will need to be modified to not call
kvm_apic_get_nr_lvt_entries() until after it has verified the local APIC is in-kernel,
but IMO that's a good thing.

>  {
> -	return KVM_APIC_MAX_NR_LVT_ENTRIES;
> +	return KVM_APIC_MAX_NR_LVT_ENTRIES - !kvm_is_cmci_supported(vcpu);
>  }
>  
>  void kvm_apic_set_version(struct kvm_vcpu *vcpu)