Re: [RFC PATCH v2 1/5] KVM: SVM: Stop warning if x2AVIC feature bit alone is enabled

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 16, 2025, Mario Limonciello wrote:
> On 9/16/25 2:14 AM, Naveen N Rao wrote:
> > On Mon, Sep 15, 2025 at 01:04:56PM -0700, Sean Christopherson wrote:
> > > On Thu, Sep 04, 2025, Naveen N Rao (AMD) wrote:
> > > > A platform can choose to disable AVIC by turning off the AVIC CPUID
> > > > feature bit, while keeping x2AVIC CPUID feature bit enabled to indicate
> > > > AVIC support for the x2APIC MSR interface. Since this is a valid
> > > > configuration, stop printing a warning.
> > > > 
> > > > Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
> > > > ---
> > > >   arch/x86/kvm/svm/avic.c | 8 +-------
> > > >   1 file changed, 1 insertion(+), 7 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > > > index a34c5c3b164e..346cd23a43a9 100644
> > > > --- a/arch/x86/kvm/svm/avic.c
> > > > +++ b/arch/x86/kvm/svm/avic.c
> > > > @@ -1101,14 +1101,8 @@ bool avic_hardware_setup(void)
> > > >   	if (!npt_enabled)
> > > >   		return false;
> > > > -	/* AVIC is a prerequisite for x2AVIC. */
> > > > -	if (!boot_cpu_has(X86_FEATURE_AVIC) && !force_avic) {
> > > > -		if (boot_cpu_has(X86_FEATURE_X2AVIC)) {
> > > > -			pr_warn(FW_BUG "Cannot support x2AVIC due to AVIC is disabled");
> > > > -			pr_warn(FW_BUG "Try enable AVIC using force_avic option");
> > > 
> > > I agree with the existing code, KVM should treat this as a firmware bug, where
> > > "firmware" could also be the host VMM.  AIUI, x2AVIC can't actualy work without
> > > AVIC support, so enumerating x2AVIC without AVIC is pointless and unexpected.
> > 
> > There are platforms where this is the case though:
> > 
> > $ cpuid -1 -l 0x8000000A | grep -i avic
> >        AVIC: AMD virtual interrupt controller  = false
> >        X2AVIC: virtualized X2APIC              = true
> >        extended LVT AVIC access changes        = true
> > 
> > The above is from Zen 4 (Phoenix), and my primary concern is that we
> > will start printing a warning by default. Besides, there isn't much a
> > user can do here (except start using force_avic, which will taint the
> > kernel). Maybe we can warn only if AVIC is being explicitly enabled?

Uh, get that platform to not ship with a broken setup?

> I'd say if you need to say something downgrade it to info instead and not
> mark it as firmware bug.

How is the above not a "firmware" bug?