Re: [PATCH v2 2/5] KVM: SVM: allow to force AVIC to be enabled

[Sean Christopherson <seanjc@google.com>]

On Mon, Dec 13, 2021, Maxim Levitsky wrote:
> Apparently on some systems AVIC is disabled in CPUID but still usable.
> 
> Allow the user to override the CPUID if the user is willing to
> take the risk.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/svm/svm.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index c9668a3b51011..468cc385c35f0 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -206,6 +206,9 @@ module_param(tsc_scaling, int, 0444);
>  static bool avic;
>  module_param(avic, bool, 0444);
>  
> +static bool force_avic;
> +module_param_unsafe(force_avic, bool, 0444);
> +
>  bool __read_mostly dump_invalid_vmcb;
>  module_param(dump_invalid_vmcb, bool, 0644);
>  
> @@ -4656,10 +4659,14 @@ static __init int svm_hardware_setup(void)
>  			nrips = false;
>  	}
>  
> -	enable_apicv = avic = avic && npt_enabled && boot_cpu_has(X86_FEATURE_AVIC);
> +	enable_apicv = avic = avic && npt_enabled && (boot_cpu_has(X86_FEATURE_AVIC) || force_avic);
>  
>  	if (enable_apicv) {
> -		pr_info("AVIC enabled\n");
> +		if (!boot_cpu_has(X86_FEATURE_AVIC)) {
> +			pr_warn("AVIC is not supported in CPUID but force enabled");
> +			pr_warn("Your system might crash and burn");
> +		} else

Needs curly braces, though arguably the "AVIC enabled" part should be printed
regardless of boot_cpu_has(X86_FEATURE_AVIC).

> +			pr_info("AVIC enabled\n");

This is all more than a bit terrifying, though I can see the usefuless for a
developer.  At the very least, this should taint the kernel.  This should also
probably be buried behind a Kconfig that is itself buried behind EXPERT.