linux-kernel - Re: [PATCH v3 6/6] KVM: SVM: Enable AVIC by default for Zen4+ if x2AVIC is support

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Message-ID: <aM3RoW3MzUfp-yto@google.com>
Date: Fri, 19 Sep 2025 14:56:49 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Naveen N Rao <naveen@...nel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/6] KVM: SVM: Enable AVIC by default for Zen4+ if
 x2AVIC is support

On Fri, Sep 19, 2025, Naveen N Rao wrote:
> On Thu, Sep 18, 2025 at 05:21:36PM -0700, Sean Christopherson wrote:
> > @@ -1151,6 +1170,18 @@ void avic_vcpu_unblocking(struct kvm_vcpu *vcpu)
> >  
> >  static bool __init avic_want_avic_enable(void)
> >  {
> > +	/*
> > +	 * In "auto" mode, enable AVIC by default for Zen4+ if x2AVIC is
> > +	 * supported (to avoid enabling partial support by default, and because
> > +	 * x2AVIC should be supported by all Zen4+ CPUs).  Explicitly check for
> > +	 * family 0x19 and later (Zen5+), as the kernel's synthetic ZenX flags
> > +	 * aren't inclusive of previous generations, i.e. the kernel will set
> > +	 * at most one ZenX feature flag.
> > +	 */
> > +	if (avic == AVIC_AUTO_MODE)
> > +		avic = boot_cpu_has(X86_FEATURE_X2AVIC) &&
> 
> This can use cpu_feature_enabled() as well, I think.

It could, but I'm going to leave it as boot_cpu_has() for now, purely because
the existing code uses boot_cpu_has() for X2AVIC and mixing the two adds
"complexity" where none exists.

I'm definitely not opposed to using cpu_feature_enabled() in general, just not
in this case (of course, we could just swap them all, but meh, it's init code).