Re: [PATCH 1/2] KVM: x86: Apply runtime updates to current CPUID during KVM_SET_CPUID{,2}

[Igor Mammedov <imammedo@redhat.com>]

On Mon,  1 Dec 2025 17:50:48 -0800
Sean Christopherson <seanjc@google.com> wrote:

> When handling KVM_SET_CPUID{,2}, do runtime CPUID updates on the vCPU's
> current CPUID (and caps) prior to swapping in the incoming CPUID state so
> that KVM doesn't lose pending updates if the incoming CPUID is rejected,
> and to prevent a false failure on the equality check.
> 
> Note, runtime updates are unconditionally performed on the incoming/new
> CPUID (and associated caps), i.e. clearing the dirty flag won't negatively
> affect the new CPUID.
> 
> Fixes: 93da6af3ae56 ("KVM: x86: Defer runtime updates of dynamic CPUID bits until CPUID emulation")
> Reported-by: Igor Mammedov <imammedo@redhat.com>
> Closes: https://lore.kernel.org/all/20251128123202.68424a95@imammedo
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Acked-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>

> ---
>  arch/x86/kvm/cpuid.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index d563a948318b..88a5426674a1 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -509,11 +509,18 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
>  	u32 vcpu_caps[NR_KVM_CPU_CAPS];
>  	int r;
>  
> +	/*
> +	 * Apply pending runtime CPUID updates to the current CPUID entries to
> +	 * avoid false positives due to mismatches on KVM-owned feature flags.
> +	 */
> +	if (vcpu->arch.cpuid_dynamic_bits_dirty)
> +		kvm_update_cpuid_runtime(vcpu);
> +
>  	/*
>  	 * Swap the existing (old) entries with the incoming (new) entries in
>  	 * order to massage the new entries, e.g. to account for dynamic bits
> -	 * that KVM controls, without clobbering the current guest CPUID, which
> -	 * KVM needs to preserve in order to unwind on failure.
> +	 * that KVM controls, without losing the current guest CPUID, which KVM
> +	 * needs to preserve in order to unwind on failure.
>  	 *
>  	 * Similarly, save the vCPU's current cpu_caps so that the capabilities
>  	 * can be updated alongside the CPUID entries when performing runtime