Re: [RFC PATCH v2 5/5] KVM: SVM: Enable AVIC by default from Zen 4

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 04, 2025, Naveen N Rao (AMD) wrote:
> AVIC and x2AVIC are fully functional since Zen 4, with no known hardware
> errata. Enable it by default on those processors, but allow users to
> continue passing 'avic' module parameter to explicitly enable/disable
> AVIC.
> 
> Convert 'avic' to an integer to be able to identify if the user has
> asked to explicitly enable or disable AVIC. By default, 'avic' is
> initialized to -1 and AVIC is enabled if Zen 4+ processor is detected
> (and other dependencies are satisfied).
> 
> So as not to break existing usage of 'avic' which was a boolean, switch
> to using module_param_cb() and use existing callbacks which expose this
> field as a boolean (so users can still continue to pass 'avic=on' or
> 'avic=off') but sets an integer value.
> 
> Finally, stop warning about missing HvInUseWrAllowed on SNP-enabled
> systems if trying to enable AVIC by default so as not to spam the kernel
> log.

Printing once on a module load isn't spam.

> Users who specifically care about AVIC

Which we're trying to make "everyone" by enabling AVIC by default (even though
it's conditional).  The only thing that should care about the "auto" behavior is
the code that needs to resolve "auto", everything else should act as if "avic" is
a pure boolean.

> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 9fe1fd709458..6bd5079a01f1 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -1095,8 +1095,13 @@ void avic_vcpu_unblocking(struct kvm_vcpu *vcpu)
>   */
>  void avic_hardware_setup(bool force_avic)
>  {
> +	bool default_avic = (avic == -1);

We should treat any negative value as "auto", otherwise I think the semantics get
a bit weird, e.g. -1 == auto, but -2 == on, which isn't very intuitive.

>  	bool enable = false;
>  
> +	/* Enable AVIC by default from Zen 4 */
> +	if (default_avic)
> +		avic = boot_cpu_data.x86 > 0x19 || cpu_feature_enabled(X86_FEATURE_ZEN4);
> +
>  	if (!avic || !npt_enabled)
>  		goto out;